Add option to hide AlarmClock Icon in StatusBar

If somebody does not need to be reminded by an icon in the status bar
of an enabled alarm. You can switch it off.

Patch Set 4: Use default preferences instead of AlarmClock.PREFERENCES

Patch Set 5: Use PreferenceManager instead of static variable +
cosmetic changes

Patch Set 6: Fix strings and address code review comments

Patch Set 7: Implement settings set/read per Nebkat's suggestion

Patch Set 8: Fix minor nitpick

Patch Set 9: Fix NPE

Patch Set 12: Fix Wrong Patch Set 11
		- Use correct DefaultPreferences instead of unused
AlarmClock.PREFERENCES

Patch Set 13: Removed obsolete null-check

Change-Id: I1de4d2b084e00118d6036e05ce7dc171f73ff438