Unhibernate an app when a user exempts it

When an app is exempted from hibernation, we unhibernate as well. This
prevents confusing situations where a user may return to the "Unused
Apps" page and sees that the app is still there even after exemption.

Bug: 205641871
Test: atest HibernationSwitchPreferenceControllerTest
Change-Id: I4f98a2212eefdb6c456a0de36d428021b262cca3