Refactor TermuxService

- Dedicated functions have been created for various actions and commands.
- The `startForeground()` call will be made both on `onCreate()` and `onStartCommand().
- The `stopForeground()` call will be made before every `onSelf()` call.
- The references to `TermuxActivity` will now be removed in `onUnbind()` as well if activity `onDestroy()` failed to do so.
- Appropriate log entries are added to help debug issues.

Author: agnostic-apollo