Bug 959440 - Various cleanups in MP4Reader. r=kinetik

Change PlatformDecoderModule::Create*Decoder() to take an
mp4_demuxer::{Audio,Video}DecoderConfig parameter, instead of enumerating all
parameters. This means the platform decoders can have more data if need be,
like the AACAudioConfig.

Change MediaDataDecoder::Input() to take an nsAutoPtr<MP4Sample>&. The sample
will be deleted by the caller (MP4Reader) if Input() returns DECODE_STATUS_OK,
but if the MediaDataDecoder wants to assume responsibility of the
lifecycle of the sample (say to enqueue it), it can forget() on the
nsAutoPtr& passed in and assume responsibility.

Call PlatformDecoderModule::Create() on the decode thread. This is a step
towards making these classes decode-thread only.

Add PlatformDecoderModule::Init(), which caches the pref's we need, since
PlatformDecoderModule::Create() is no longer called on the main thread, we can
no longer access them in there.

Add Init() method to MediaDataDecoder interface. This is so that we can call
MediaDataDecoder::Shutdown() to unblock the initialization of a decoder, if
that init needs to block.

Pass LayersBackend type to WMFVideoDecoder, so it knows whether to init DXVA.
Bug 959440 - Apply MP4Reader cleanups to WMFDecoderModule. r=padenot

Update WMFDecoderModule and friends to reflect changes to PlatformDecoderModule
interface in previous patch.