Fix mono gapless playback for decoders that output stereo

The old AAC decoder always outputs stereo, even for mono source material, so we
need to use the number of channels of the output when calculating the number of
bytes to skip, not the number of channels in the source.
This makes OMXCodec skip the right amount of data, and prevents NuPlayer from
writing half a frame and then asserting when the AudioSink doesn't accept it.
Also move use of the SkipCutBuffer from NuPlayer to ACodec, so that it also
works when using the new Java APIs, and make SkipCutBuffer derive from RefBase.
b/774846

Change-Id: I34df9fea3e6730617eae559afaa556f4085ef0a0