[BR11] Read data saver status from bpf

Writing data saver status to bpf is supported on Android V or
later devices. Thus, read that from bpf if available.

Test: atest FrameworksNetTests:android.net.connectivity.android.net.BpfNetMapsReaderTest
Test: atest ConnectivityCoverageTests:android.net.connectivity.android.net.ConnectivityManagerTest
Fix: 310801259
Change-Id: Ibd2616328d83f72ee6d2665239c3a44379d1ebf5