stagefright: Fix port-reconfiguration & output-buffer-filled race condition

Remove the invalid assumption that when a port-reconfiguration event
is received, buffers cannot be downstream (waiting to be rendered).

Luckily, these buffers are properly handled (freed) after they are
sent to be rendered.

Change-Id: I1df39c1ffc2bfb96f8b7b4ee5be07cae654f956f
Signed-off-by: Lajos Molnar <lajos@google.com>
Bug: 8736466