stagefright: Clear RenderQueue on port settings changed

Clear the RenderTracker RenderQueue before allocating new set of
buffers during port settings change. The graphicBuffers inside
the RenderQueue hold the actual buffer references which prevents
them from being freed until this queue is cleared.
Do not wait till executing state to clear the queue as then
overall memory consumption goes up for the brief period.

CRs-Fixed: 972394
Change-Id: If50ffc2d517f793a59c88d3fb213bf4c23b8c9f7