stagefright: Fix thumbnail generation for some clips.

Fix thumbnail generation for clips that include an immediate resolution change:
* Enable dynamic resolution change, when port settings may change while we have
  filled buffers.
* Handle the case when port settings change happens right after stop is called.

Bug: 16210021
Change-Id: I8b2c48365d74ce7551f000a1f4b0711647a9796f