Set -bootclasspath to "" if LOCAL_NO_STANDARD_LIBRARIES

Without this if a Java libraryw with LOCAL_NO_STANDARD_LIBRARIES doesn't
have core-libart in its LOCAL_JAVA_LIBRARIES, it would use core-libart
as bootclasspath but without having dependency on it.

Change-Id: I611a5a5df2bf0ad59e0be50ed1ad1afe8b843e90