Add the label attribute on TermuxActivity

Should fix the F-Droid naming of the app, see
https://github.com/termux/termux-app/issues/1107

app/src/main/AndroidManifest.xml

@@ -30,6 +30,7 @@
 
         <activity
             android:name="com.termux.app.TermuxActivity"
+            android:label="@string/application_name"
             android:configChanges="keyboard|keyboardHidden|orientation|screenSize"
             android:launchMode="singleTask"
             android:resizeableActivity="true"