Add shadow mode.

Add a mode in which the TARE system can go through the motions and
pretend it's doing work without actually affecting client component
behavior.

Bug: 158300259
Test: atest frameworks/base/services/tests/mockingservicestests/src/com/android/server/tare
Test: atest frameworks/base/services/tests/servicestests/src/com/android/server/tare
Test: atest FrameworksMockingServicesTests:AlarmManagerServiceTest
Change-Id: I9eda78ca2437bab23701e7d2d666e5e58b9964cd