Pin objenesis to Java language level 8.

These jars are loaded by an OpenJDK 8 runtime from
prebuilts/studio/jdk in frameworks/layoutlib/bridge/tests/run_tests.sh
so the upcoming change of the platform default to Java language level
9 will break those tests.

This change can be reverted if/when prebuilts/studio/jdk is upgraded
to OpenJDK 9+.

Test: EXPERIMENTAL_JAVA_LANGUAGE_LEVEL_9=true make layoutlib layoutlib-tests sdk libandroid_runtime
Test: frameworks/layoutlib/bridge/tests/run_tests.sh out/layoutlib-failures
Bug: 141970724
Change-Id: Ic9eb1ce0bea0310f70d63935a342c1312e2b834d
diff --git a/Android.bp b/Android.bp
index a37e050..b00600f 100644
--- a/Android.bp
+++ b/Android.bp
@@ -20,6 +20,11 @@
     hostdex: true,
     srcs: ["main/src/main/java/**/*.java"],
     sdk_version: "core_current",
+
+    // Pin to Java language level 8. It is loaded by an OpenJDK 8 runtime from
+    // prebuilts/studio/jdk in frameworks/layoutlib/bridge/tests/run_tests.sh.
+    // See b/141970724.
+    java_version: "1.8",
 }
 
 // Compatibility library for old name of host target