Fixes AOSP crash when WPP isn't installed.

If there is no installed application with the ability to start an
activity with the SET_WALLPAPER action, we no longer show the
"Shortcuts" entry under the "Display > Lock screen" settings page.

Flag: CUSTOMIZABLE_LOCK_SCREEN_QUICK_AFFORDANCES
Fix: 274075866
Test: expanded unit test
Test: manually verified that, if Intent.resolveActivity returns false,
the shortcuts item isn't present in settings.

Change-Id: I919427d1e71a88a520c762307029a53065a68f46