Fix bootclasspath for host variants of java_library modules

The partial hostdex support was causing the host variant of
java_library modules to depend on core-oj and core-libart,
which caused the tagsoup jar to use the wrong
java.lang.System.arraycopy signature.  Remove the hostdex
code that was causing the problem, and add a test.

Test: java_test.go
Change-Id: I4f7b1f29c99aae328ba19b042538d9d35544aa43