Move WebView loader from frameworks/webview/ to frameworks/base/.

The Java-side of the WebView loading lives in frameworks/base/ while the
native side lives in frameworks/webview/. It would be great to be able
to change the JNI interface between these two without having to update
two separate projects.

This CL moves the native side into frameworks/base/.

Bug: 62445369
Test: Run app using WebView (and ensure it loads WebView).
Change-Id: I6915e996b3a035e9d87000ccd11e5fb89deecde7