Shortcut Helper - Retrieve app categories shortcuts from WindowManager

Before, we were hard coding the shortcuts. Now there is a new API from
WindowManager to retrieve these shortcuts, which are defined in a
config file.

Fixes: 341045436
Test: Manual
Test: AppCategoriesShortcutsSourceTest.kt
Flag: com.android.systemui.keyboard_shortcut_helper_rewrite
Change-Id: I2d3234129c37b777e4e18e213f7f9af0ee8d781a