Fix Progressive Battery Saver's Data Saver toggle.

Make sure that Data Saver can be enabled properly with Progressive
Battery Saver. Before this change, NetworkPolicyManagerService would
look at the global battery saver enabled flag, which meant that Data
Saver would only be enabled if BatterySaverPolicy.advertiseIsEnabled
and BatterySaverPolicy.enableDataSaver were both set to true. Now it
only requires the latter.

Bug: 119261320
Test: atest com.android.server.net.NetworkPolicyManagerServiceTest
Change-Id: I0f538129c4ed6a586316426a35e767d2d6b6ba1d