LPP will now always launch either GlobalActions or the Assistant.

Previously, on devices that default to launching Assistant
on long-press power, toggling this checkbox in Settings
would switch between that default (launching Assistant) and
a no-op. Those devices now toggle between Assistant and
GlobalActions (the power menu), just like devices that have
the old default behavior of LPP launching GlobalActions.

Change-Id: Id36031e70825da195f9c30ce05b3ae3d5e49b5fb
Test: make RunSettingsRoboTests \
	ROBOTEST_FILTER=LongPressPowerButtonPreferenceControllerTest
Bug: 192946325
(cherry picked from commit 509670cb)