[3FT] "Open another app" Preference

Add an extra action to the list that opens a placeholder page

Bug: 406792393
Flag: com.android.settings.flags.touchpad_settings_design_update
Flag: com.android.settings.flags.three_finger_tap_app_launch
Test: in follow up CLs
Change-Id: I77b7ec81450d7c27a56157b0c9f314f926e3e16f