Set assistant as a role

- Register a role observer in VoiceInteractionManagerService. Once the
role is changes map the new role setting onto the old settings.
- As the assistant role is now always set, there is no need to have code
in AssistUtil for the case the assistant setting is not set
- Remove old config option for the default assistant. The default
assistant is not configured via the roles config

Bug: 110557011
Test: - Set, unset and swtiched assistant via the settings UI
        - for voice interaction service
	- for assist activity
      - Booted from freshly wiped device and saw assitant to be set to
        default
Change-Id: I8596f49c6f6496e8b70cf3236aaa7d7557443a93