No-op refactor on EmergencyGesturePreferenceController.

Move custom intent launching logic into a separate controller
(EmergencyGestureEntrypointPreferenceController).

This is in preparation of a mini-IA refresh and fixing a search
bug. The custom intent logic is only needed by the entrypoint
pref, and putting in the gesture PrefControler makes any fix
hard to implement.

Bug: 171067360
Bug: 172609395
Test: RunSettingsRoboTests
Change-Id: I1e26393872bfada98db80a1cc33e3e13dacb086b