Stop wifi display discovery when no longer needed.

Keep track of how many clients are requesting scans and scan
continuously until all of them are gone then explicitly terminate the
scan instead of letting it time out as before.

Suspend wifi display scans while connecting or connected to a remote
display.  This is handled by both the display manager and media router
since neither has complete information about what is happening.
Much of this code will no longer be needed once wifi display support
is integrated directly into the media router service.

Ensure that we don't attempt to scan or connect to wifi displays
while the wifi display feature is off.

Infer when a connection attempt fails and unselect the wifi display
route automatically so it doesn't appear to be connecting forever.

Fix issues around correctly canceling and retrying connection attempts.
Often we would cancel but not retry.

Improved connection reliability somewhat.  It seems that discovery must
already be in progress in order for a connection attempt to succeed.

Ensure QuickSettings uses exactly the same logic as the MediaRouteButton
to determine when the remote display tile should be made visible.

Bug: 11717053
Change-Id: I18afc977b0e8c26204b8c96adaa79f05225f7b6e