Fix App permission doesn't show summary

Since AppPermissionsPreferenceController assigns
a fixed preference key in constructor, so it can't
be used by same controller in Privacy page.

So, we change contructor directly, let controller
read preference key from xml file.

- Also remove unused imports.

Test: visual
Bug: 116628158
Change-Id: I42d6e978e91f0a2fe796d290cb9f34db596aa1f4