Add a FakeSharedPreferences for testing

 * apply and commit both act in the same way, applying the contents of
   the Editor synchronously
 * data is stored in memory in the instance
 * listeners are dispatched synchronously on the same thread that calls
   apply/commit
Test: atest FakeSharedPreferencesTest
Bug: 180126509

Change-Id: Ie443f6269a9aa6d699631d8524bfa9fe6b3474e4