Strip relative paths from java_import output files

androidx.annotation_annotation is used as a test data file, and
converting it from a java_library to a java_library_import causes
the relative path used in the test data path to change. Clear the
relative path in java_import the same way that other java based
modules do.

Bug: 288358614
Test: TestJavaLibraryOutputFileRel
Change-Id: I1f494110da32e916043ca94ac6ebeeafccc06f9a