PlaybackThread: Request real-time priority on ARC

MixerThread handles audio mixing and it implements PlaybackThread.
1. In ARC++, nice value of a thread will be divided by 2 so
THREAD_PRIORITY_URGENT_AUDIO is not enough for PlaybackThread.
2. In our experiments, the threads density per core in ChromeOS
are generally more than normal Android. The latency under using CFS
scheduler with any priority is not enough for MixerThread to process
audio data in time.

We request the lowest real-time priority, SCHED_FIFO=1, for
PlaybackThread in ARC++.

This is to upstream ARC-only CL ag/24624964, by checking
ro.boot.container and only requesting the real-time priority on ARC.

For the context of the original CL, see ag/5604522.

Bug: 301627581
Test: 1. Build this CL on main-arc-dev branch and flash to ARCVM
      2. `ps -AT -eo rtprio,comm,name`, check AudioOut rtprio is 1
Change-Id: If9aece940fce31722a188a8629f3d167813d4205