Properly handle executing->idle in GraphicBufferSource

ACodec has initiated the transition from exectuing->idle and is waiting
for the component to return all buffers so they can be freed.
It's counter productive and with some vendors harmful to submit additional
buffers to the component from this point on.

Change-Id: I332be254a1faf94902c77b5792a373ab447e44d3
related-to-bug: 11573543