ConsumerBase: discardFreeBuffers() also needs to dump its own cache

ConsumerBase has its own cached slots with graphic buffer references,
so it's not enough to just ask the buffer queue consumer to free
buffers.

Add code equivalent to what happens in the onBuffersReleased callback.

Test:
Bug: 62593139
Change-Id: Ibc1444b868c6150aa2da1c209e06bdba42f1595d