bp2build converts java_version property to javacopts attribute

This CL also converts `external/rappor` (which already set `java_version` to `1.7`) to be bazelable to testify the changes.

Results from `b build //external/rappor && cat bazel-bin/external/rappor/librappor.jar-0.params`: https://paste.googleplex.com/5518725462622208.

Test: go test ./bp2build/...
Bug: 227618664
Change-Id: I8d370d4639f70fba51e6de6ceb7bcb5ace9ccd91

bp2build/java_proto_conversion_test.go

@@ -102,6 +102,7 @@ func TestJavaProtoDefault(t *testing.T) {
 		blueprint: `java_library_static {
     name: "java-protos",
     srcs: ["a.proto"],
+    java_version: "7",
 }
 `,
 		expectedBazelTargets: []string{
@@ -115,7 +116,8 @@ func TestJavaProtoDefault(t *testing.T) {
 					"deps": `[":java-protos_proto"]`,
 				}),
 			makeBazelTarget("java_library", "java-protos", attrNameToString{
-				"exports": `[":java-protos_java_proto_lite"]`,
+				"exports":   `[":java-protos_java_proto_lite"]`,
+				"javacopts": `["-source 1.7 -target 1.7"]`,
 			}),
 		},
 	})