Optimize out extra jar copies

Don't copy jar files for disabled stages.  Instead, set the name of
the output to the name of the input so the next stage will directly
pick up the output of the previous stage.

Test: m -j ANDROID_COMPILE_WITH_JACK=false java
Change-Id: Ib7268cbff7ea7ff2ad2caf994aa145e1b7a12b83