AudioTrack checks the channel configuration against a mask of allowed channels.
Before Android M the check in getMinBufferSize would always fail for 8 channel
output.
For Android L constants are available to build the required channel
configuration for a conventional 7.1 setup and these constants are part of the
allowed channels mask, so switch to using them.
Before Android L the constants for 7.1 aren't in the allowed channels mask, so
discard channels to give a 5.1 stream.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=206307892
