Prettify the captive portal sign-in activity

- fuses url bar with activity header: url is now the subtitle
- url appears as text and not as editable content
- url only shows host name
- header style is the device default for Settings
- Material progress bar
- progress bar disappears at page load finished
- webview does not jump at page load finished

Bug: 62107381
Bug: 38197949
Test: tested by manually triggering the sign-in activity
Change-Id: Id75023acad04f444dbfc41de56814a09e70cc6f9