Extract common methods to abstract class (2/N)

Prepare for Activity snapshot controller.

Extract mapping:
TaskSnapshotCache => AbsAppSnapshotCache
TaskSnapshotController => AbsAppSnapshotController
TaskSnaspshotPersister => BaseAppSnapshotPersister
Rename TaskSnapshotLoader to AppSnapshotLoader.

Bug: 259497289
Test: build/flash, verify snapshot can be save/load from disk.
Test: atest TaskSnapshotPersisterLoaderTest TaskSnapshotCacheTest
TaskSnapshotLowResDisabledTest

Change-Id: I6c7454fb628f75fefad99063b25f37436526c7a0