#3 broad rework of mix-mode compilation and fixes.

[hsyed]

Converted some of our internal projects to mixed-mode, this pr contains fixes and refactors.

• For Mixed-Mode compilation - Compile Java and Kotlin in one pass in the worker. #3 Annotation Processing #6 generate classes to a directory in the sandbox instead of a temp directory.
• Two phase compilation. Don't let kotlinc compile java. Kotlinc wrapping of javac has issues with the bazel use case.
• Add simple javac wrapper to handle java compilation.
• Kotlinc will generate indirect errors on broken Java. So if kotlinc fails compile java anyway and issue the java errors first.
• Allign flags with JavaBuilder.