Plumb MediaCodecRenderer.enableDecoderFallback through DefaultRenderersFactory

[ojw28]

[REQUIRED] Use case description

Enable decoder fallback without having to subclass DefaultRenderersFactory, as requested
here.

@andrewlewis - Is there a reason we didn't expose the possibility of fallback for the audio renderer as well?

[andrewlewis]

As discussed I don't think there's a reason not to expose fallback for audio (I was going on the assumption that audio decoders are generally not hardware-accelerated so are much less likely to fail to initialize transiently). I will do that as part of this enhancement.