Allow ConnectivityServiceTest to change the calling UID.

Allow ConnectivityServiceTest to change the UID by replacing
static calls to Binder.getCallingUid() with a method that can
be mocked.

Add registerNetworkCallbackAsUid as an initial way to exercise
this, and add some test coverage to the always-on lockdown test
to confirm that things are working as expected.

Bug: 173331190
Test: new unit tests
Change-Id: Ie0b32460e20e5906a0f479191e11a062f21cc608