Clean up, simplify and fix AlarmTile class and logic

This commit cleans up the AlarmTile class by removing the unnecessary
listener on the alarm intent. It now listens only to the next alarm
setting value, which was fetched after receiving the intent anyway.

These changes also fix a bug that would cause the tile to not show up
after some actions, like reordering the tiles due to the visibility not
being set on tile creation.

Lastly, some minor code styling issues have been fixed.

Patch set #2: collapse onNextAlarmChanged into onChangeUri, remove
mEnabled, use TextUtils to check for string emptiness and mark
updateQuickSettings explicitly as public.

Change-Id: Ic5871a9cc4874f33181dfac14e25a89499c182f2