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

java/java.go

@@ -2707,8 +2707,13 @@ func (m *Library) convertJavaResourcesAttributes(ctx android.TopDownMutatorConte
 	var resources bazel.LabelList
 	var resourceStripPrefix *string
 
+	if m.properties.Java_resources != nil && len(m.properties.Java_resource_dirs) > 0 {
+		ctx.ModuleErrorf("bp2build doesn't support both java_resources and java_resource_dirs being set on the same module.")
+	}
+
 	if m.properties.Java_resources != nil {
 		resources.Append(android.BazelLabelForModuleSrc(ctx, m.properties.Java_resources))
+		resourceStripPrefix = proptools.StringPtr(ctx.ModuleDir())
 	}
 
 	//TODO(b/179889880) handle case where glob includes files outside package