Flush binder buffer after setting raw heap to avoid leaking a reference.

The problem was:

1. In handleShutter(), thread A in CameraService calls
   registerBuffers(IMemoryHeap) and it's received by thread B
   in system_server. [transaction 1]
2. While thread A is waiting for the reply, thread B calls
   back to thread A to get the id of the heap
   (IMemoryHeap.getHeapID). [transaction 2]
3. Thread A replies transaction 2 and is preemptied in kernel.
   Thread B gets the reply and finishes registerBuffers and send
   reply for transaction 1.
4. When thread A runs again, it gets the reply for transaction 1
   and returns to handleShutter().
5. At this point the transaction buffer for transaction 2 (which
   holds a reference to IMemoryHeap) is not freed because the
   BC_FREE_BUFFER command is kept in thread A's local command
   queue and not sent to the kernel.
6. Normally when thread A makes next transaction, the
   BC_FREE_BUFFER command will be sent together (piggyback) with
   the commands for that transaction. But in this case thread A
   is a callback thread from camera driver, so it does not make
   any binder calls afterwards, and the IMemoryHeap is never freed
   (until the next time handleShutter is called).

Change-Id: I435a258187509bdbbaf353339eb9ea577610cbd2