Clean up handling of NetworkCapabilities#administratorUids.

Update ConnectivityService's check for administrator UIDs to use
ArrayUtils to check for UID inclusion. Update the NetworkCapabilities
annotation on the administrator UIDs field to clarify that it is
NonNull.

Bug: 147903575
Test: atest FrameworksNetTests
Change-Id: Id630fe9d76aacdaf038fdaa5360f0327520ee0c3