Added mergeWithNextTransaction to BlastBufferQueue

Resurrected abandoned cl Ife2a8c9dbcfac7a5c3b294f64bc8dce40231f652

This is to support SurfaceView synchronizing transactions with
ViewRootImpl. Currently, when SurfaceView gets a callback from
PositionUpdateListener, it asks ViewRootImpl whether it's
waiting on a sync transactions. If so, it will add the SV transactions
to the sync transactions. Otherwise, it will apply the transaction on
its own.

By adding the new function mergeWithNextTransaction, SV can call into
BlastBufferQueue and send its own transaction without having to know
whether VRI is in a blast sync. As long as blast is enabled, it can
always send its transaction to the BlastBufferQueue, which will apply it
when the VRI frame comes in.

Test: open bubbles with sv blast
Test: atest SurfaceViewSyncTest
Bug: 175594838
Change-Id: I1d1b6eb3bbcaa844f430dd3a2f7cfbe59e558909