Remove onSyncComplete from SurfaceSyncGroup

There was already a way to register for an entire SurfaceSyncGroup
completing. There just wasn't a way to tell what SurfaceSyncGroup
something was added to in order to register in VRI. Therefore, add the
addedToSync argument in the readyToSync callback, allowing VRI to
register a complete callback instead of exposing a new API.

Also renamed onReadyToSync to onAddedToSyncGroup to reflect what is
happening. Renamed SyncBufferCallback to TransactionReadyCallback since
there isn't a guarantee the transaction contains a buffer.

Test: SurfaceSyncGroupTest
Test: NotificationShade Sync
Bug: 237804605
Change-Id: Ib58735cd86593a976e3f4559997316275bd82cd9