Invoke sync transaction callback if overwritten

If a caller invokes the syncNextTransaction it will overwrite any
callback that was set before. This can break since the caller expects to
receive the callback.

The change invokes the old callback when a new callback overwrites it.
It also invokes the callback when the blast buffer queue is torn down in
case there's one still pending.

Test: Chrome in split + rotation
Test: BLASTBufferQueueTest
Fixes: 223329500
Change-Id: I91d1d7c47f4eca014c66ba8c2f773ef8be5c674e