libstagefright: optimize thumbnail generation path.

- Checking for YUV420SP format support is not intended for software
  decoders and can be skipped. This avoids unnecessary creation of
  hardware decoders for checking supported output formats.
- Change the flag from kPreferSoftwareCodecs to kSoftwareCodecsOnly.
  This will force software codecs to be tried first and not just "prefer"
  which helps when a software codec for a particular type of codec (like
  divx) doesn't exist and avods creation of hardware codecs twice in case
  of hardware failure.

Change-Id: I682bca5b6bac5a56f46fe3655cd722fe5f08e1e6