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.

Differently from commit 2e0915f1, the
zooming buttons are not displayed (i.e zooming is possible only with
gesture).

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