Make it easier to test code that uses WakeupMessage.

Code that uses WakeupMessage uses the AlarmManager. Testing such
code is slow because AlarmManager.MIN_FUTURITY ensures that
alarms must wait at least 5 seconds before firing.

This change makes WakeupMessage's fields protected so that test
code can subclass from it and override schedule() and cancel()
with implementations that do not use AlarmManager, for example
by making schedule() call sendEmptyMessageDelayed and making
cancel() call removeMessages.

Change-Id: I51096b182d9eb87cc7bd46c3c91906f18356b354