Add permission justifications to AppPermissionFragment.

This adds text describing an app's permission justifications to
AppPermissionFragment.

Bug: 63532550
Bug: 111207567
Test: Open AppPermissionFragment, see text.
Test: Manually hardcode different values, see text change.
Test: atest android.appsecurity.cts.PermissionsHostTest
Change-Id: I7873227e49fd276eb7bc7639e8883c0124d7c73b