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

java/java.go

@@ -2089,6 +2089,11 @@ func (m *Library) convertLibraryAttrsBp2Build(ctx android.TopDownMutatorContext)
 	if m.properties.Javacflags != nil {
 		javacopts = append(javacopts, m.properties.Javacflags...)
 	}
+	if m.properties.Java_version != nil {
+		javaVersion := normalizeJavaVersion(ctx, *m.properties.Java_version).String()
+		javacopts = append(javacopts, fmt.Sprintf("-source %s -target %s", javaVersion, javaVersion))
+	}
+
 	epEnabled := m.properties.Errorprone.Enabled
 	//TODO(b/227504307) add configuration that depends on RUN_ERROR_PRONE environment variable
 	if Bool(epEnabled) {