Use new app preload path for WebView zygote.

In the "common" case where the WebView stub is not being used, have the
WebView zygote use the new APK preload path added for the app zygote,
which avoids duplicating logic from the framework to construct the
classpath. This allows the WebView implementation to use a static shared
library, which was not previously possible.

The old codepath is retained for now to keep the WebView stub working
when it's in use, as it requires the special mechanism to override the
classloader cache key, but this can be removed when we no longer require
the stub.

Bug: 110790153
Test: Manual verification of classloader cache state
Change-Id: Ie49e5810d570bae7bec0341753e6c50d081189b5