Start AccessCheckingService in SystemServer.

The service constructor and onStart() only does trivial work and its
start time is also tracked by TimingsTraceAndSlog. The new app op and
permission implementation will only be active when they are later
selected inside the existing permission and app op system services and
that selection will happen behind a feature flag disabled by default.

Bug: 182523293
Test: presubmit
Change-Id: I2d678ef2d5747440afebc92868f92c4a02565856