[FSD2D] Add BackupManager API to start restore

Add an API to BackupManager to start restore in migration mode.

Bug: 160407842
Test: atest PerformUnifiedRestoreTaskTest BackupManagerServiceTest
            UserBackupManagerServiceTest
Change-Id: I75fdb1c99edc3d11d1264d69e1f20c682d588479
10 files changed