CaptivePortal: login activity UI improvements

Similarly to commit 2e0915f1 for the
carrier portal login, this patch changes the default settings of the
webview used for the system captive portal login activity to allow
better user experience on "wide" login pages designed for desktop.

Test: manually tested with captive portals.
Bug: 31813936, 19228946, 36532213
Change-Id: Ib84fd351e47e951d24f297bc7de1b035b51cf24f
Merged-In: I2579994da37f3b0f4c08e24e59c81f31835ab832

(cherry picked from commit f7eea406)