Ensure CachedAppOptimizerThread cpuset

CachedAppOptimizerThread isn't started until the device config is
downloaded from the Phenotype server, but CachedAppOptimizer.init() is
called at device boot.

Configure the system-background cpuset for CachedOptimizerThread at
creation, then set it again when the thread is started, to ensure
priorities aren't modified by ActivityManager.

Test: flashed/wiped the device and verified no optimizer thread was
present until wifi connection, then verified that the thread cpuset was
system-background once created. Rebooted the device again and verified
that the optimizer thread was system-background cpuset again.
Bug: 149610535

Change-Id: I310f234552320f95bd7e05053084425a3fcc6dcf