Add configuration to control appearance of long press power to assistant setting

This configuration will control whether Settings app will display the setting to allow the user to switch long press on power behaviour from configured default to invocation of assistant app. In case of existing default being configured to assistant option (5) already, the setting will toglee between Assistant and power menu/global actions.

Bug: 179175321
Bug: 182983853
Test: Configuration XML update only.
Change-Id: Ia220f34c305d5a7c4e54fa9f5ceded096db98103