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_library_conversion_test.go

@@ -158,6 +158,22 @@ java_plugin {
 	})
 }
 
+func TestJavaLibraryJavaVersion(t *testing.T) {
+	runJavaLibraryTestCase(t, bp2buildTestCase{
+		blueprint: `java_library {
+    name: "java-lib-1",
+    srcs: ["a.java"],
+    java_version: "11",
+}`,
+		expectedBazelTargets: []string{
+			makeBazelTarget("java_library", "java-lib-1", attrNameToString{
+				"srcs":      `["a.java"]`,
+				"javacopts": `["-source 11 -target 11"]`,
+			}),
+		},
+	})
+}
+
 func TestJavaLibraryErrorproneJavacflagsEnabledManually(t *testing.T) {
 	runJavaLibraryTestCase(t, bp2buildTestCase{
 		blueprint: `java_library {