Validate objects are from the correct context.

Change-Id: I7d87b0e253b8d2e36d1aed790cfe3a7dd23e158f

rs/java/android/renderscript/RenderScript.java

@@ -1004,6 +1004,14 @@ public class RenderScript {
         }
     }
 
+    void validateObject(BaseObj o) {
+        if (o != null) {
+            if (o.mRS != this) {
+                throw new RSIllegalArgumentException("Attempting to use an object across contexts.");
+            }
+        }
+    }
+
     void validate() {
         if (mContext == 0) {
             throw new RSInvalidStateException("Calling RS with no Context active.");