Add utility interface for WebView preparation logic.

To make the WebView preparation mechanism testable we add a utility
interface that can be overridden during a test to avoid calling the
Android framework and to provide custom WebView packages.

With this change we also split some of the code from the WebViewFactory
(code unrelated to WebView loading) into a separate utility class.

Bug: 27635535
Change-Id: I265ecd42b24ad5383637e125b3654ff339c9df9c