Filter shortcuts in the widget picker activity.

In its current state the WidgetPickerActivity only allows adding widget
items, so to match the state, updated it to show only widgets.

manual test - https://screen/cast/NDUxMjg1NjI0MTM0MDQxNnwxMzE4MzNhOS1mZg

Bug: 320495335
Test: atest Launcher3Tests
Flag: N/A
Change-Id: Ifa75f457219efef49e104efc428d369187db2710