Fix CS external dependencies on CarrierDefaultApp

- Use formal API getSystemService to get ConnectivityManager.
- Stop using setProcessDefaultNetworkForHostResolution(), it's
  only used for legacy API startUsingNetworkFeature() but
  CaptivePortalLoginActivity is used requestNetwork(). So remove
  it from CaptivePortalLoginActivity.

Bug: 183068713
Test: atest CarrierDefaultAppUnitTests
Change-Id: Ib3a5dcb70b71ec5b959aeb90a5e3596f3426ceb1