From DataUsageSummaryPreferenceController.
NetworkPolicyEditor.read() is not called, so
NetworkPolicyEditor.getPolicy() will always returns null.
Data limit info is already set in getDataUsageInfo().
Bug: 295260929
Test: robo test
Change-Id: I56a40cc906aa95207785886e704b1a59ea344e9c
Created a new controller for DataUsageInfo and add appropriate
unit tests.
Bug: 30946416
Change-Id: Ida1281a771013fd807242f846edb4f2a9c0ccabb
Test: Robolectric tests with target RunSettingsRoboTests