Remove PreL/PostL implementation of AlarmClockFragment.

Bug: 17425204

Created a TimePickerCompat that displays either post-L version
TimePickerFragment or pre-L version, and always retains callback via
setTargetFragment in order to survive screen rotation.

This makes the inheritance chain shorter for alarm related fragments,
and encapsulates the display logic for timer picker.

Change-Id: I6951305854f61221cca72805922a427c01d7320d