Store tag filter persistently when App is stopped, e.g. with force stop ...

Fixed a problem in OI Notepad that would cause the currently selected tag filter to get lost when the app was force-closed or the user pressed the back button on the device. Previously the selected tag filter was also lost when the user exited from editing a note by pressing the "Edit note" button at the top left of the edit note activity. As a result, OI Notepad did always switch back to the "All notes" view. For me this was very annoying because I mostly work in one category of notes and don't want to see others most of the time.

This problem occurred in OI Notepad 1.3 and 1.4.0.7.

The fix stores the active tag filter persistently to survive even when the app is stopped or the device is powered off.

I tried to limit the changes to a minimum. Merging should be straigh forward.