Allow the System UID to unregister ConnDiags CBs.

This CL updates ConnectivityService to allow the System's UID to
unregister ConnectivityDiagnostics callbacks. Preivously, only the
registrant was allowed to unregister them - this caused problems for
callbacks that were attempted to be unregistered via binderDied() when
the registrant app dies.

Bug: 159912975
Bug: 174713659
Test: manually verified
Change-Id: I20d0cad5f902708d366aa703c2893b0ea3e55052