stagefright: Be more tolerant of missing metadata for FFMPEG codecs

 * If these codecs are instantiated programatically and required
   metadata isn't sent, just set some defaults instead of crashing on
   an assert.
 * This fixes testAllNonTunneledVideoCodecsSupportFlexibleYUV in MR1 CTS

Change-Id: I69bf6105a1be529298de574bd5d3b6813e7a4e8f