Rework voice interaction session lifecycle.

We now have a formal concept of the session being shown and
hidden, with it being able to continue running while hidden
as long as there is enough RAM.

This changes the flow that a VoiceInteractionSession will
see: onCreate() is when it is first created, onCreateContentView()
is when its UI first needs to be built, onShow() is called each
time it needs to be shown and has the arguments given when the
show request was made (which has been renamed from startSession to
showSession), and then onHide() will be called when the UI is
no longer shown.

The methods show() and hide() now allow a VoiceInteractionSession
subclass to control when it is shown and hidden, working with the
shown state being maintained by the system.

Change-Id: Ic4a430ec7e8bf76a5441fd0425e2932806170fcc