Captive portals: login activity probes like NetworkMonitor

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
Change-Id: I33cbcdab9809c712d998da6d925bdd4e3093f8ff