Add flag to voice interactor for supporting assist gesture

Add an additional flag to the voice-interaction-service declaration
so it can indicate whether it is able to handle the assist gesture.
Use that information in SystemUI so it only starts the voice
interaction session if the service is able to support it.

Change-Id: I62b035ce4f4cf06ee6e7eb0ddc4bf5edbc0e6737
9 files changed