Use java language level 8 for compiling module stubs

This is in line with the main stubs, which use 8. I believe using 9 is
unsuitable because it could generate bytecode that is incompatible with
some developer tooling stuck on 8. Either way, being consistent makes
sense.

Bug: 144149403
Test: diff_stubs.sh
Change-Id: Iddeceeebe9175e53e34f4b4194f01992e3083532

apex/Android.bp

@@ -176,6 +176,7 @@ java_defaults {
     installable: false,
     sdk_version: "module_current",
     libs: [ "stub-annotations" ],
+    java_version: "1.8",
     dist: {
         targets: ["sdk", "win_sdk"],
         dir: "apistubs/android/public",
@@ -187,6 +188,7 @@ java_defaults {
     installable: false,
     sdk_version: "module_current",
     libs: [ "stub-annotations" ],
+    java_version: "1.8",
     dist: {
         targets: ["sdk", "win_sdk"],
         dir: "apistubs/android/system",
@@ -198,6 +200,7 @@ java_defaults {
     installable: false,
     sdk_version: "module_current",
     libs: [ "stub-annotations" ],
+    java_version: "1.8",
     dist: {
         targets: ["sdk", "win_sdk"],
         dir: "apistubs/android/module-lib",