Add new VoiceInteractionSession.setUiEnabled method

This allows the session to turn off all of its UI.  Use it
in onCreate() to prevent the session from ever creating any
of its UI.  Switched the window creation and initialization to
happen lazily to support this.

Test: New test added to android.assist.cts.LifecycleTest
Bug: 30999386

Change-Id: I8eacb3697c8ac9908c50b9577abea2d20597b016