Fixes activity leak in Wallpaper Picker.

Fix: 280060800
Test: unit tests expanded
Test: manually verified that long-pressing an affordance icon brings up
its configuration activity.
Test: manually verified that tapping the button in an enablement dialog
that's shown when the user clicks on a disabled affordance icon opens
its target activity.

Change-Id: Ie36566d777276458531f304dfd82aa2ca6a86379