Replace SyncTarget with SurfaceSyncGroup

Remove SyncTarget and make all uses of SyncTarget use SurfaceSyncGroup.
This makes sense moving forward because we want to add SurfaceSyncGroups
to other SurfaceSyncGroups.

The flow will be the following:
1. Create a SurfaceSyncGroup
2. Add that SurfaceSyncGroup to another SurfaceSyncGroup if it needs to
   be combined with other things
3. When no more things need to be added to the SurfaceSyncGroup and it's
   ready to finish, call onTransactionReady. This will mark the
   SurfaceSyncGroup as ready and allow it to apply or merge if it has no
   pending SurfaceSyncGroup children

This works well in VRI since we can just create a single
SurfaceSyncGroup to represent the VRI itself. SV can add their own
SurfaceSyncGroups to the VRI's one. VRI's SurfaceSyncGroup can be added
to any other SurfaceSyncGroups that want to synchronize the VRI. VRI
just calls onTransactionReady directly on its SurfaceSyncGroup when it
has drawn its buffer. If it has a parent, it will be notified and the
transaction merged into the parent. If no parent, it will be applied
directly.

Test: SurfaceSyncGroupTest
Test: SurfaceSyncGroupContinuousTest
Bug: 237804605
Change-Id: If691412505e8ba2503dd093399227327a0b6fdb1