Add new methods for setting mobile data policy

Add isMobileDataPolicyEnabled and setMobileDataPolicyEnabledStatus in
TelephonyManager. These are intended to replace the mms-always-allowed
and data-during-voice-call setters/getters.

Old methods will be removed in a follow up commit.
Fixes: 169367013
Test: atest TelephonyManagerTest

Change-Id: I6033a84f0a0163e460343ebf603d66078455fe38