This patch changes CaptivePortalLoginActivity captive portal test to be
consistent with NetworkMonitor by:
- using Network.java to open the http connection.
- adding a UserAgent property to the request header.
Test: manually tested.
Bug: 32369183
(cherry picked from commit 454e066e27)
Change-Id: I559eb0497475daad758ba3b3395225dcd0a27a57