Implement firewall status bar indicator

* Add API to get the last resumed activity's UID
* Show blocked status indicator based on the last resumed activity's
  UID blocked status
* Rely on NetworkCallbacks from ConnectivityManager instead of network
  policy callbacks from INetworkPolicyListener to react to blocked
  status changes rather than policy changes

Co-authored-by: Michael Bestas <mkbestas@gmail.com>
Change-Id2: Ic5ba608ed468ae50dba5e92a4350b2cd7a190221
Change-Id: I7aa405e7f5a2f70e6cf2b6156a29d220617e4ae5