Pin staledataclass-annotation-processor to Java language level 8.

This target tries to use --add-modules and --add-exports to get access
to internal classes from the jdk.compiler module. This works for Java
code (unsupportedappusage-annotation-processor) but not for Kotlin
code, as here. The kotlinc invocation fails.

As a workaround for this issue, this change pins this to Java language
level 8, so the JPMS doesn't apply.

Test: EXPERIMENTAL_JAVA_LANGUAGE_LEVEL_9=true make staledataclass-annotation-processor
Bug: 139342589
Change-Id: I5234bd3daf5f5347547ead50445ba989b38cbf6e

tools/processors/staledataclass/Android.bp

@@ -13,6 +13,8 @@ java_plugin {
     static_libs: [
         "codegen-version-info",
     ],
+    // The --add-modules/exports flags below don't work for kotlinc yet, so pin this module to Java language level 8 (see b/139342589):
+    java_version: "1.8",
     openjdk9: {
         javacflags: [
             "--add-modules=jdk.compiler",