Fix incorrect handling of single permission groups

The system incorrectly handles the situation where a permission group is
comprised of a single permission, by treating it as if it was a normal
permission group. Added special hanlding for these cases.

Fixes: 139440264
Test: Have a single permission permission group (located under the
"Additional permissions" screen in the permission controller. Click
a permission group, then click an app in this group. The app permission
screen should appear.

Change-Id: I7213a6373fd40ff98649a587ee1a6c34eb8f2d77