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Remove unsafe GenericPlayer::getSampleRate().  getSampleRate() was not
thread-safe as it accessed a member variable updated in the looper thread
without a lock.  If getSampleRate() is ever needed again, the sample rate
is available in the mPcmFormatValues array, which does have a lock.

Replace member variables mChannelMask, mChannelCount, and mSampleRateHz
by local variables for locality and performance.

Change-Id: Ibf92ed090430153d748ded4d0b835e429df47eea