[BR07.1] Expose setDataSaverEnabled from ConnectivityManager

Currently, data saver switch is controlled in NPMS, which
calls into NetworkManagementService and netd when switching
status. In netd, BandwidthController manipulates the
bw_data_saver chain to control the overall behavior.

However, this code are all platform implementation, which
are not updatable. In order to migrate data saver switch
from iptables to bpf, this API is needed for Connectivity to
change the implementation to directly write bpf in later
patches.

Test: atest CtsHostsideNetworkTests:com.android.cts.net.HostsideRestrictBackgroundNetworkTests
Test: atest FrameworksServicesTests:NetworkManagementServiceTest
Bug: 297836825
Change-Id: I71fa41ca739fef8e191fba91b02758ad5f732d5c