Source jars were not working as designed because javac will only
compile files from the -sourcepath if there are references to them
starting from files on the command line. Switch to extracting
the source jars into a directory and passing a list of the files
to javac.
Test: m checkbuild
Change-Id: I9f7d824f8538d081b2f5ad64ae3cbfd0e96213af
