[Local Screen Casting] Observe displays only when querying for sinks

This CL makes the WiredDisplayMRP observe displays only after
MediaRouter requests to query for sinks, so that we are not observing
displays during startup and looking up the primary display, which is
causing a crash in display-less Chrome OS.

Bug: 803881
Change-Id: I751d1cd295fa3db1c1bdee65570ee4ab7ad8cc23
Reviewed-on: https://chromium-review.googlesource.com/924524


Commit-Queue: Takumi Fujimoto <takumif@chromium.org>
Reviewed-by: Derek Cheng <imcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537926}