CCodecBufferChannel: stash flushed input buffers from PipelineWatcher

Currently converter code reconstructs a buffer object when it comes
through HAL interface. Using this buffer in flushed work could
confuse lifecycle management of the buffers, so retrieve the input
buffers from the PipelineWatcher which stores the original input
buffer references.

Bug: 196014695
Test: cts/media/device-small
Change-Id: I06128438d0de52e139e939afe7e59b166f404c1a