Provide a resource_prefix_strip for java_resources in bp2build.

Although paths to resource files in a Bazel java_library should be
relative to the package, the directory structure in the resulting jar
will have resources under the full path from the top-level of the
workspace, e.g. if a library in "a/BUILD" has java_resouces as
"res/res.txt" then by default the res.txt file would appear under
"a/res/res.txt".
Fix this by adding a resource_strip_prefix in that case.

Test: Unit tests
Change-Id: If4325126f5c19a2a8fb83ee09bc3a95a18673fe3