Implement communal widget filtering based on category filter setting.

A new setting in Settings allows filtering communal widgets to either
"keyguard" or "all". This change uses that setting to pass the correct
filter key to Launcher when showing the widget picker activity.

Bug: 314804170
Test: atest SystemUiRoboTests
Flag: ACONFIG com.android.systemui.communal_hub DEVELOPMENT

Change-Id: I2d6cd4bc783ebd4f2b69a750e6f7276751830517