Move kotlinc-build.xml out of classes directory

kotlinc-build.xml was ending up in the final jar because it was
being created in the directory where the classes were written.
Rename the variable containing the classes directory from outDir
to classesDir to discourage putting other files in there, and
move kotlinc-build.xml up a directory.

Bug: 114524493
Test: m checkbuild
Change-Id: Id2e2af0a6543499bd25b8fbd642bc59b14bd241f