Improve java_sdk_library handling of test_current

When building a library against sdk_version: system_current it uses the
system stubs of any java_sdk_library that it references. Previously,
when building against sdk_version: test_current this used the public
stubs of any java_sdk_library. This change causes it to use the
test stubs instead to be consistent with the handling of system and
public.

Bug: 148080325
Test: updated the test
      ran m nothing which failed as expected
	  fixed the code
	  ran m nothing again which succeeded
Change-Id: I58ec5bd243701c5a5c75664e2bb615ce7b2f2441