Ensure @GuardedBy errors are flagged

Make the relevant Error Prone checks errors rather than warnings, so
if they detect any inconsistency in our locking, we find out about it.

Bug: 258396046
Test: Introduce bug, see error
Change-Id: I2edffb24ad7bc9ea55975bd17b66f1e8233af3d2

javalib/Android.bp

@@ -31,6 +31,13 @@ java_sdk_library {
         // android.sysprop.*, renamed by jarjar
         "com.android.system.virtualmachine.sysprop",
     ],
+    errorprone: {
+        // We use @GuardedBy and we want a test failure if our locking isn't consistent with it.
+        enabled: true,
+        javacflags: [
+            "-Xep:GuardedBy:ERROR",
+        ],
+    },
 }
 
 prebuilt_apis {