Decrement networkRequestPerUid when callbacks are unregistered.

ConnectivityDiagnosticsCallbacks are tied to NetworkRequestInfo objects
when registered with the platform. Each NetworkRequestInfo is tied to a
specific uid, and ConnectivityService enforces a limit on the number of
network requests that can be associated with each uid.

When ConnectivityDiagnosticsCallbacks are unregistered from the
platform, their NetworkRequestInfo is freed and the number of network
requests per the user's uid should be decremented.

Bug: 150802582
Test: atest android.net.cts.ConnectivityDiagnosticsManagerTest
Change-Id: Ia5ed39c1d8e6221cd402be4f8baf69fa643a6113