Add gatherPendingTransactions in BBQ

Add a function to gather the transactions that were sent to BBQ via
the mergeWithNextTransaction. This is to allow the caller to get the
transaction in case a frame isn't drawn and we need to apply or merge
these transactions anyway.

Test: Builds and runs
Bug: 200284684
Change-Id: Ic9f3afb48e00212a6488a6a8f9cd4403ed1cb7d6