Add new permission / appop for reading loader data

Bug: 148374455
Test: Manual
Test: Create three packages; 1) only request PACKAGE_USAGE_STATS, 2) only request LOADER_USAGE_STATS, 3) request both
Test: Go to Settings -> Apps & Notifications -> Special app access
Test: See that there is only a single entry for "Usage access"
Test: Under "Usage access", see all three test apps appear
Test: Disable access for each application and verify that only the appop for the declared permisson is flipped using
Test: adb shell cmd appops get <<PACKAGE_NAME>>
Test: atest CtsPermission2TestCases
Change-Id: I7741a703fd4494832347e51e113adf974cc31d2b