Update layoutlib to match java library changes

javalib.jar is now only used for jars that contain dex files, so
host java libraries are now always called classes.jar.  Update
layoutlib's custom rules to match.  Also remove built_ext_data,
the classes.jar in built_ext_classes already contains the
resources.

Bug: 36902714
Test: layoutlib-tests
Change-Id: I522d9e3786ad7deeb291c7c1a61ec4b86be03dc2
(cherry picked from commit 15d18782)