Added exit/close button and related strings so you can exit app without forcing it to stop
Summary
Adds a button to the BackgroundService notification when there is no device paired so the user can close the app without forcing it to stop.
Also adds a button to the app drawer for the same purpose.
Since this is my first commit, and I'm not familiar with native android programming, I didn't let the user interact with both buttons when there is a device paired, because I don't know what needs to be done to gracefully exit the app in this case.
Test Plan
Probable tests could be:
Pair with a device and verify that both buttons are not available to interact;
Verify that the required actions to gracefully exit the app are finished before exiting the app,