Move Termux Properties to com.termux.app.settings.properties package

The termux properties handling was mixed in with termux preferences. They are now moved out of into a separate sub package, the following classes are added:

- `SharedProperties` class which is an implementation similar to android's `SharedPreferences` interface for reading from ".properties" files which also maintains an in-memory cache for the key/value pairs. Two types of in-memory cache maps are maintained, one for the literal `String` values found in the file for the keys and an additional one that stores (near) primitive `Object` values for internal use by the caller. Write support is currently not implemented, but may be added if we provide users a GUI to modify the properties. We cannot just overwrite the ".properties" files, since comments also exits, so in-place editing would be required.
- `SharedPropertiesParser` interface that the caller of `SharedProperties` must implement. It is currently only used to map `String` values to internal `Object` values.
- `TermuxPropertyConstants` class that defines shared constants of the properties used by Termux app and its plugins. This class should be imported by other termux plugin apps instead of copying and defining their own constants.
- `TermuxSharedProperties` class that acts as manager for handling termux properties. It implements the `SharedPropertiesParser` interface and acts as the wrapper for the `SharedProperties` class.

app/build.gradle

@@ -11,6 +11,7 @@ android {
         implementation "androidx.viewpager:viewpager:1.0.0"
         implementation "androidx.drawerlayout:drawerlayout:1.1.1"
         implementation 'androidx.core:core:1.5.0-beta02'
+        implementation 'com.google.guava:guava:24.1-jre'
         implementation project(":terminal-view")
     }