Use local defined constants for notification channels

ConnectivityService mainline module cannot call the members
inside SystemNotificationChannels.java. So use the local defined
constants instead.

Bug: 172040168
Test: atest FrameworksNetTests
Test: Manual test NO_INTERNET, SIGN_IN, PARTIAL_CONNECTIVITY,
      VPN_ALWAYS_ON notifications.
Change-Id: I223b207c4346e8239dee78e50dd2b0532a3e55b0