Add hidden API getDataSaverViaCmFlag() for CTS

The CtsHostsideNetworkTests:com.android.cts.net.
HostsideNetworkCallbackTests needs to know whether trunk flag is enabled
or not. But there is no stable way to read the flag from CTS for module.
So adds this hidden API in platform.

Bug: 288340533
Test: atest
CtsHostsideNetworkTests: com.android.cts.net.HostsideNetworkCallbackTests

Change-Id: I02cea917a3f75f11d6ea5754a6ea0d460f1372c9