Check MOBILE_DATA_POLICY_MMS_ALWAYS_ALLOWED

To know whether MMS is always allowed, should check
isMobileDataPolicyEnabled(MOBILE_DATA_POLICY_MMS_ALWAYS_ALLOWED)
instead of isDataEnabledForApn(ApnSetting.TYPE_MMS).

Fix: 303759893
Test: manual - toggle "MMS messages" switch
Test: unit test
Change-Id: Id0417e569ff4e3dbac1ecff9327b4a964125c122