Added Bugfix Flag to guard App Shortcut Removal Fix.

Adding flag to guard this Fix ag/32498740

Also modified test so that it fails with flag off and passes with flag
on - This makes more sense.

Test: CustomShortcutCategoriesRepositoryTest
Flag: com.android.systemui.app_shortcut_removal_fix
Fix: 405058325
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:c92830fb84aa243989f431a4dd5a03cfeb26309a)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:ed6b6d67dd70db84c7654979b91ae214615748bf)
Merged-In: I90c3361b75cacecac8045d3061fb0e588c5b3661
Change-Id: I90c3361b75cacecac8045d3061fb0e588c5b3661