Update SyncStorageEngine to use protos.

Status and Statistics info within SyncStorageEngine was being stored
using Parcels, which is not recommended. Updating both of them to use
protos now. Additionally, because of protos, storage space on disk
consumed by these files are reduced by more than 50%.

Also added unit tests for SyncStorageEngine.

Bug: 38177679
Test: atest com.android.server.content.SyncStorageEngineTest
Test: build and flash - no errors in logs
Change-Id: I187cb6c271333f366f5c7f651f75b4161fee90d4