Add WifiConfiguration.shared flag

* Add boolean WifiConfiguration.shared, which indicates whether a network
  is shared (visible to and usable by all users) or private (visible to
  and usable only by the user who created it)
* Extend WifiConfiguration.configKey(...) so that when two users create
  private configurations for the same network, their configurationKeys
  will be different.
* Make AccessPoint.matches(...) take into account the shared flag

BUG=25600871

Change-Id: I4a4335fa3b4b4f850e16748583a29ab66992ddc8