Add gatherPendingTransactions in BBQ

Instead of applying the pending transactions when a frame wasn't
rendered, we gather the transactions and merge them with the transaction
that will be used to sync. This is to ensure these frames are sent to
the caller that requested the sync so they can ensure it's applied at
the correct time.

Test: Builds and run
Bug: 200284684
Change-Id: I087c67138f31cc93e79df42c29464ab4f7989d52