PermissionCache caches permission checks

This is intended to absorb the cost of the IPC
to the permission controller.
Cached permission checks cost about 3us, while
full blown ones are two orders of magnitude slower.

CAVEAT: PermissionCache can only handle system
permissions safely for now, because the cache is
not purged upon global permission changes.

Change-Id: I8b8a5e71e191e3c01e8f792f253c379190eee62e