Add an API to listen for changes in network blocked status of an uid.

Given that ConnectivityService is moving to a mainline module, we need
a @SystemApi for it to listen for changes in blocked status of an uid.
So, we decided to create a new API for this which can provide
ConnectivityService with more info about why an uid is blocked (which
will be useful for adding a new similar onBlockedStatusChanged callback
in CM.NetworkCallback) and also captures data saver restriction without
having out to track it separately.

Currently, NPMS does some redundant computations because we are
calculating both uid rules and blocked reasons separately. In a
follow-up change, we will compute uid rules using blocked reasons
and later possibly remove that onUidRulesChanged callback.

Bug: 176289731
Test: atest ./tests/cts/hostside/src/com/android/cts/net/HostsideRestrictBackgroundNetworkTests.java
Test: atest ./tests/net/java/com/android/server/ConnectivityServiceTest.java
Test: atest ./services/tests/servicestests/src/com/android/server/net/NetworkPolicyManagerServiceTest.java
Change-Id: Ib9949b8619c6b148f73630b314c4113d76c31ec1
Merged-In: Ib9949b8619c6b148f73630b314c4113d76c31ec1