Add SurfaceComposerClient::Transaction clear api

When a client parcels a transaction, it is desirable to clear the transaction object since applying the
transaction multiple times by different clients may not make sense.
Also fixes SurfaceComposerClient::Transaction merge api to clear states like mForceSynchronous when
clearing the object.

Bug: 132205507
Test: go/wm-smoke
Test: test preserve surfaces codepath with split screen and ensure relative z is preserved
Change-Id: Iaedb3d420804ff70089817d239273c381b7ebc31