Remove NetworkPolicyManager.isUidBlocked() API.

It isn't used by ConnectivityService any more and even if
it needs such utility method in the future, we could create
one which is part of connectivity module and doesn't need
to be exposed as part of NetworkPolicyManager API surface.

Bug: 183696103
Test: atest ./tests/net/java/com/android/server/ConnectivityServiceTest.java
Change-Id: Ie3c681f88e4b2b9bb92d2224c5ea96b074f155d5