Add a method for fetching the WebView classloader.

The WebView Support Library needs a way to hook into the WebView APK,
this new method returns a classloader that can be used to load internal
WebView classes.
The idea is to have this classloader be provided by the WebView APK
itself, so that the classes accessed from the classloader can be
filtered to avoid apps accidentally adding dependencies on internal
details of the WebView APK.

Test: N/A - cannot add CTS tests until we drop a WebView APK containing
the implementation for this API.
Bug: 38220806
Change-Id: If744a740ff0b1728d2d4674e73697bc6a5e27dde