Show current WebView package as selected in Dev Setting.

When intenting to the select-webview-implementation Dev Setting a user
won't know which package is currently being used as WebView
implementation. To solve this we change the background color of the item
representing the current WebView package.

With this CL, also ensure we update the summary of the WebView
implementation Dev Setting even if we failed to switch to the package
chosen by the user - we might have switched to another package instead.

Bug: 34702900
Test: Ensure the current WebView package has a different background
color than the rest of the packages.

Change-Id: Ieb65fdc635d5dd1253931c8ab8bb0d827a142c56