Clean up LOCAL_DONT_DELETE_JAR_META_INF

Allow setting it for host modules, it is necessary for
RoboLectric.

Remove some unused PRIVATE_DONT_DELETE_JAR_META_INF variables.

Remove unzip-jar-files, it is obsolete since merge_zips was
introduced.

Test: m checkbuild
Change-Id: I3f2fc466321904eace31525cbac99ad524f39188