stagefright: OMX.ffmpeg.* are software decoders, ensure they're treated ...

...as such

Wherever the component name for OMX.google soft decoders is used to identify
a software-based component, do the same for ffmpeg. Things like memory management
and window buffers care about this.

Change-Id: Ib83561936c7383e8726edb073cea9d78f7d1312f