[BR06] Support check whether network is blocked by data saver

This change adds a DataSaverStatusTracker, which is a helper
class to continuously track data saver status through NPMS
public API and intents. ConnectivityManager#isUidNetworkingBlocked
would use this cached information along with bpf maps to decide
whether networking of an uid is blocked.

Test: atest FrameworksNetTests:android.net.connectivity.android.net.BpfNetMapsReaderTest
Test: atest ConnectivityCoverageTests:android.net.connectivity.android.net.ConnectivityManagerTest
Bug: 297836825
Change-Id: I7e13191759430f3ea1f4dec7facc02f16be7146d