Scheduling group cleanup

Remove C++ APIs androidSetThreadSchedulingGroup and
androidGetThreadSchedulingGroup, and the ANDROID_TGROUP_* constants.

Former callers of these should now use the C APIs set_sched_policy and
get_sched_policy, and the SP_* constants.

Note: debug.sys.noschedgroups is not supported by the C APIs,
this needs to be discussed.

Change-Id: I32bbfc539ef4090faf9ef0320380e8cca9eae07c