Enable migration to big cores for app launches.

Uses cpusets to move all foreground tasks to the big cores in order
to improve overall app launch latency. Big cores will be used for
three seconds, and then the cpuset assignment is reset, allowing
foreground tasks to fall back to the little cores as appropriate.

Associated system/core and device/* changes in order to enable
the boost cpuset and configure it per-device.

bug 21915482

Change-Id: Id8a0efcb31950c1988f20273ac01c89c8c948eaf