Pass output file names into java.Transform* functions

Pass the output file name into the java.Transform* functions.
This consistently puts control of the filename into java.go,
which is often necessary to avoid collisions when the same
rule is used multiple times in a single module.  It also
has the side-effect of removing the poorly named "stem"
parameters.

Test: java_test.go
Change-Id: I7bc1d1f3bfae6f9d2c92870e6df381817817aab4