Add an item to "Legal information" screen for WebView license

Similar to how it is done for other legal information items --
if there is an activity that serves android.settings.WEBVIEW_LICENSE
intent, the item is shown with the title set to the activity's title,
otherwise the item is hidden.

Bug: 18729447
Change-Id: Ic5bad40c91e35fa3c8235128628413929ef111d3