Don't do a wifi p2p scan in MediaRouter if we're already connected

When we're connected to a wifi display already, some configurations
might hiccup or otherwise disrupt the media stream if we initiate a
new scan. Block the MediaRouter's default scanning behavior if we
start up while already connected to a wifi display. (A scan will still
occur if the user opens the picker dialog.)

Change-Id: I413efa883eec41fe0377da18bcb55bc72e2ac729