Get dex jar resources from classpath jar

Dex jars were getting their resources from the res.jar files of
their transitive static dependencies.  This accidentally bypassed
jarjar on resources, since the jarjar pass only happened once the
resources jar was combined into the classpath jar.  Switch to
getting the resources out of the classpath jar by merging it
with the dex jar while skipping *.class.

Test: m -j checkbuild
Test: compare ext.jar to one generated by make
Change-Id: I5f6f3da738dcb0af56ab9a1bd7174ed5359de2b2