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.
The failsafe activity were infrequently used while confusing users.
Replace it with an app shortcut on Android 7.1+ and a separate app on
earlier versions.
