Adding SafetyCenter system service and permission.

This implements a basic version of go/angel-in-mainline-api, only
for the bits that are unlikely to change (i.e. introduce the ability
for safety sources to provide data to the safety center).

SELinux Policy changes in aosp/1897505 and aosp/1899011

Test: ag/16284943
Ignore-AOSP-First: Yes
Bug: 203098016
Bug: 203400898
Bug: 205111656
Change-Id: I5d139782decf5226f0c4a8dcaed7c5691a86fed4