Use dx.jar directly

Use $(JAVA) to invoke dx.jar instead of the dx wrapper script.
Also allow the dx command line or the dx jar to be specified on
the command line.  Allows better dependencies on dx.jar, and
doesn't rely on having java on the path.

Bug: 62956999
Test: m -j checkbuild
Change-Id: I5f265132690ad28b3139954fea44eb0c4a43393a