Prevent SIGN_IN notification pop up several times

SIGN_IN notification may pop up several times before user
logged in the network if someone trigger
ConnectivityService#reportNetworkConnectivity().
Add a boolean to make sure SIGN_IN notification will only pop up
once to user before user logged in.

Bug: b/122164725
Test: Build pass.
Change-Id: I9c7f1d47fde8e5dfadd6646497023b05f3b0ed88
Merged-In: I9c7f1d47fde8e5dfadd6646497023b05f3b0ed88