Make javac checkbuilds match jack

Building all java to dex is very slow.  Make javac checkbuilds act
like jack checkbuilds and only build to class files for unused
modules, and add a java-dex target that can be used to build
everything to dex.

Test: m -j checkbuild
Change-Id: Idcf9c9e0223b9dd88aaa53beb0cb50a230034b0f