Add manual rollback sample app

This application can be installed as a signature app. It displays
all available rollbacks, and will manually trigger the rollback of
all selected rollback IDs. This application will not trigger a
reboot in the case of staged rollbacks.

Test: m SampleRollbackApp, adb install. Manually install a train
      with rollback enabled and reboot. Verify that the available
      rollback is displayed, and that the rollback can be committed.
Bug: 220204580
Change-Id: Id2e5afac9e25532d8c24ea2b803c07dcfdb84d85