Require a system permission to open the settings activity
This branch adds a permission to SettingsActivity so it can only be opened by our own app or an app with the READ_NETWORK_USAGE_HISTORY permission, which is only granted to system apps.
This allows the system settings app to open SettingsActivity but prevents other apps from doing so (including am on the command line).
Tested on Android 4.0.4 and 6.0.1 (CM 13).
Fixes the first part of #909 (closed)