Check location permission for ConnDiags last.

This CL updates ConnectivityService to check location permissions for
ConnectivityDiagnostics callbacks last in the permission check process.
This minimizes misattribution of location access for networks that an
app is not administering.

This CL also updates ConnectivityDiagnosticsManager documentation to
clearly state that location permissions are required in order to receive
callbacks.

Bug: 187310575
Test: atest ConnectivityDiagnosticsManagerTest
Test: atest ConnectivityServiceTest
Change-Id: I2dbeddac6273e2392ccaeae51a1c7776d6d3da75