AudioMixer: Clear bufferProviders in correct order

Issue:
 While switching between clips with different track properties, when
earlier track is cleared, postDownMixerBufferProvider
tries to release a buffer to serverProxy instead of the original owner
i.e. downMixBufferProvider. This illegal releaseBuffer call to serverProxy
results in an assert in AudioTrackShared.
-In issue scenario, data flow path in AudioMixer is,
ServerProxy-->-->DownMixer-->PostDownMixer-->Resampler,
- Clear for downMixerBufferProvider ensures that all serverproxy
buffers are returned back.
-This also causes the postDownMixer to get connected with serverProxy.
-Hence on delete of postDownMixer illegal releaseBuffer for
serverProxy gets executed.
Fix:
Clear PostDownMixerBufferProvider before clearing DownMixerBufferProvider
to ensure that buffers are release to right owners.

Change-Id: I982366660d0a1e04be8cca6dabe758221dedf9b1