Keyguard Transitions - Add REVERSE option

When a transition is canceled and a new one starts, there are multiple
ways to continue from the cancellation point: RESET, LAST_VALUE, or
REVERSE. REVERSE was added in order to support a smooth transition
from AOD<->LOCKSCREEN when the power button is pushed the middle.

Fixes: 305978396
Test: atest KeyguardRepositoryImplTest KeyguardTransitionScenariosTest
Change-Id: Ie03ba8a55707cceeb69a591db1bd5fd9636164f1