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.

Also, the case where buffers have been sent onto the native window
is already handled.

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