Fix transactionCallbackThunk lifetime issue

TestBLASTBufferQueue overrides transactionCallback but BLASTBufferQueue::transactionCallback can trigger the destruction of BLASTBufferQueue, causing native crashes. This CL fixes the issue by moving the decStrong call after transactionCallback has completed.

Bug: 294133380
Flag: EXEMPT bugfix
Test: BLASTBufferQueueTest
Change-Id: Ia2344ed6d14e89026f464b32c95c76b3a96a19d0