Write Data Saver setting to BPF map

The information is needed by modules who want to know whether a
specific UID is blocked by Data Saver feature.

1. Add a one-element map data_saver_enabled_map.
2. Update current data saver setting to the map.

Bug: 288340533
Test: atest FrameworksNetTests:android.net.connectivity.com.android.serv
er.BpfNetMapsTest
Test: atest bpf_existence_test

Change-Id: I981da4b569247c33cba2d365cb6f2691f673474e