[bugfix] new TransitionDrawable memory leak

[buptfarmer]

[bugfix] new TransitionDrawable point to old drawable array, which cause memory leak

As we know, TransitionDrawable is a LayerDrawable which contains a array of two drawables need to do transition. But directly use old drawable to create new TransitionDrawable, let the the newly created TransitionDrawable hold a reference to the old array rather than the only drawable.

placeholder = imageView.getDrawable();

So I break the reference chain and fix like this:

                                Drawable oldDrawable = imageView.getDrawable();
                                if (oldDrawable != null) {
                                    if (oldDrawable instanceof LayerDrawable) {
                                        LayerDrawable oldLayerDrawable = (LayerDrawable) oldDrawable;
                                        placeholder = oldLayerDrawable.getDrawable(0);
                                    } else {
                                        placeholder = imageView.getDrawable();
                                    }
                                } 

Please review that, and I hope you may accept this pr.

[patrickfav]

Hi buptfarmer, thanks for the PR, will review in the next couple of days :)
Dont worry about the failing build, currently PR branches miss the private keys. Have to find a workaround in the future.