Replace hidden CM#isNetworkSupported() usage

It's a refactor work for connectivity mainline module. The
hidden methods access is no longer allowed, so the usage for
isNetworkSupported() should be replaced. Settingslib use it to
check if device support telephony service. Use alternative
method to check if device supports such feature.

Bug: 172183305
Test: make RunSettingsLibRoboTests
Change-Id: I713c6410fcf9543d54d9b0057ed7b2ecdab201ee