Also tid == 0 maps to gettid() on platforms that support it.
This is needed by mediaserver code that was formerly using
the old C++ API.
Change-Id: I8baaa3e42315c9b924cec26b6da01e7cb69e0078
This will make it easier to add additional policies (cgroups) if needed.
Also added comments to the sched_policy APIs.
Change-Id: I33ce1cc4deae10983241f7391294b7a512d2c47c