Correctly handle empty usages.

When PermissionUsages finds no usages (which happens at least for
custom permissions), it does not call its callback, which causes UI
screens to be waiting forever.  This ensures it calls back and has
AppPermissionUsage stop showing its loading bar.

Test: View AppPermissionUsage with a custom permission.
Test: Click around Permissions Hub a lot.
Change-Id: I19e2c914b7f54c9ff11d6edd04f5ed98f3cde58f