Added CloseGuard for BaseObj

Bug: 27719830

To turn on warnings, apps have to add to their Activity.onCreate() method
the following code.

        StrictMode.setVmPolicy(new StrictMode.VmPolicy.Builder()
                               .detectLeakedClosableObjects()
                               .penaltyLog()
                               .build());

For Slang generated ScriptC derived classes, we assume their
constructors won't throw exceptions after calling the ScriptC
constructor. In addition, ScriptIntrinsic derived classes do not seem
to throw exceptions in their constructors either. Therefore, we can
leave the guard.open() call in the Script constructor. This may be
only an approximation, but allows us to add CloseGuard for script
objects without making changes to slang.

Change-Id: I77ed45239a60b85af5c811dee6c124fb53da9060