Clean up Javac a little bit.

- Added GLOBAL_JAVAC_DEBUG_FLAGS and merge it to
  PRIVATE_JAVACFLAGS/PRIVATE_JACK_FLAGS to get rid of
  PRIVATE_JAVAC_DEBUG_FLAGS/PRIVATE_JACK_DEBUG_FLAGS.
- With Java rules out of base_rules.mk we can get rid
  of java_alternative_checked_module now.

Change-Id: I1a14716c785e3d49330a75044107662ce96a9307