Improve TestConnectivityManager readability

Remove NetworkCallbackInfo to use Handler directly, and separate
registerSystemDefaultNetworkCallback from requestNetwork.

Also verify that requestNetwork is only used with a default-like network
request on R, as S should be using registerSystemDefaultNetworkCallback.

Test: atest TetheringTests
Bug: 189581241
Change-Id: If6b9124b2d795b9db1e85256c694d0120c852542
1 file changed