Define TermuxConstants class to store all shared constants of Termux app and its plugins

This commit removes almost all hardcoded paths in Termux app and moves the references to the `TermuxConstants` class.

The `TermuxConstants` class should be imported by other termux plugin apps instead of copying and defining their own constants. The 3rd party apps can also import it for interacting with termux apps. App and sub class specific constants are defined in their own nested classes to keep them segregated from each other and for better readability.

app/src/main/AndroidManifest.xml

@@ -2,13 +2,13 @@
     xmlns:tools="http://schemas.android.com/tools"
     package="com.termux"
     android:installLocation="internalOnly"
-    android:sharedUserId="com.termux"
+    android:sharedUserId="${TERMUX_PACKAGE_NAME}"
     android:sharedUserLabel="@string/shared_user_label" >
 
     <uses-feature android:name="android.hardware.touchscreen" android:required="false" />
     <uses-feature android:name="android.software.leanback" android:required="false" />
 
-    <permission android:name="com.termux.permission.RUN_COMMAND"
+    <permission android:name="${TERMUX_PACKAGE_NAME}.permission.RUN_COMMAND"
         android:label="@string/run_command_permission_label"
         android:description="@string/run_command_permission_description"
         android:icon="@mipmap/ic_launcher"
@@ -28,13 +28,14 @@
     <uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES" />
 
     <application
-        android:extractNativeLibs="true"
-        android:allowBackup="false"
+        android:label="@string/application_name"
         android:icon="@mipmap/ic_launcher"
         android:roundIcon="@mipmap/ic_launcher_round"
         android:banner="@drawable/banner"
-        android:label="@string/application_name"
         android:theme="@style/Theme.Termux"
+
+        android:extractNativeLibs="true"
+        android:allowBackup="false"
         android:supportsRtl="false" >
 
         <!-- This (or rather, value 2.1 or higher) is needed to make the Samsung Galaxy S8
@@ -42,7 +43,7 @@
         <meta-data android:name="android.max_aspect" android:value="10.0" />
 
         <activity
-            android:name="com.termux.app.TermuxActivity"
+            android:name=".app.TermuxActivity"
 	    android:label="@string/application_name"
 	    android:configChanges="orientation|screenSize|smallestScreenSize|density|screenLayout|uiMode|keyboard|keyboardHidden|navigation"
             android:launchMode="singleTask"
@@ -60,7 +61,7 @@
         </activity>
 
         <activity
-            android:name="com.termux.app.TermuxHelpActivity"
+            android:name=".app.TermuxHelpActivity"
             android:exported="false"
             android:theme="@android:style/Theme.Material.Light.DarkActionBar"
             android:parentActivityName=".app.TermuxActivity"
@@ -68,9 +69,9 @@
             android:label="@string/application_name" />
 
         <activity
-            android:name="com.termux.filepicker.TermuxFileReceiverActivity"
+            android:name=".filepicker.TermuxFileReceiverActivity"
             android:label="@string/application_name"
-            android:taskAffinity="com.termux.filereceiver"
+            android:taskAffinity="${TERMUX_PACKAGE_NAME}.filereceiver"
             android:excludeFromRecents="true"
             android:resizeableActivity="true"
             android:noHistory="true">
@@ -100,7 +101,7 @@
 
         <activity-alias
             android:name=".HomeActivity"
-            android:targetActivity="com.termux.app.TermuxActivity">
+            android:targetActivity=".app.TermuxActivity">
 
             <!-- Launch activity automatically on boot on Android Things devices -->
             <intent-filter>
@@ -112,7 +113,7 @@
 
         <provider
             android:name=".filepicker.TermuxDocumentsProvider"
-            android:authorities="com.termux.documents"
+            android:authorities="${TERMUX_PACKAGE_NAME}.documents"
             android:grantUriPermissions="true"
             android:exported="true"
             android:permission="android.permission.MANAGE_DOCUMENTS">
@@ -122,25 +123,25 @@
         </provider>
 
         <service
-            android:name="com.termux.app.TermuxService"
+            android:name=".app.TermuxService"
             android:exported="false" />
 
         <service
             android:name=".app.RunCommandService"
             android:exported="true"
-            android:permission="com.termux.permission.RUN_COMMAND" >
+            android:permission="${TERMUX_PACKAGE_NAME}.permission.RUN_COMMAND" >
             <intent-filter>
-                <action android:name="com.termux.RUN_COMMAND" />
+                <action android:name="${TERMUX_PACKAGE_NAME}.RUN_COMMAND" />
             </intent-filter>
         </service>
 
         <receiver android:name=".app.TermuxOpenReceiver" />
 
-        <provider android:authorities="com.termux.files"
+        <provider android:authorities="${TERMUX_PACKAGE_NAME}.files"
                   android:readPermission="android.permission.permRead"
                   android:exported="true"
                   android:grantUriPermissions="true"
-                  android:name="com.termux.app.TermuxOpenReceiver$ContentProvider" />
+                  android:name=".app.TermuxOpenReceiver$ContentProvider" />
 	<meta-data android:name="com.sec.android.support.multiwindow" android:value="true" />
 	<meta-data android:name="com.samsung.android.multidisplay.keep_process_alive" android:value="true"/>
     </application>