Add PermissionGrantRequestResultReported atom

Also add a prototype for a future system-api accessible metrics file
that will be auto-generated from atoms.proto

Test: - ./out/host/linux-x86/bin/statsd_testdrive -p com.google.android.permissoncontroller 170
      - triggered permission request
Bug: 123594188, 123663448
Change-Id: Icede6ff1f12ca79ebad6267c045a4fb3a9955402