Make ImageDecoder return animated Drawables

Bug: 63909536
Bug: 63908092
Test: TODO

If ImageDecoder.decodeDrawable is called with an animated image Source
(currently GIF or WebP), return an object of a new (hidden) Drawable
subclass. The new Drawable animates, and it implements Animatable (TODO:
implement Animatable2) so users have some control over the animation.

In addition to the normal features of Drawable, this new one supports
many of the features of ImageDecoder, including scaling, cropping and
PostProcess, which enables circle masks/rounded corners and other
arbitrary after-effects. It does *not* support decoding directly to a
Hardware Bitmap, since it cycles through frames and reuses the same
bitmap memory. But it could be made to use shared memory (TODO?).

TODO: Use a better number for the native allocation registry
TODO: Use the RenderThread to drive the animation, and remove decoding
on the UI thread.
TODO: Add support for modifying the loop count

Android.bp:
- build new AnimatedImageDrawable.cpp

AndroidRuntime.cpp:
- register new native methods

AnimatedImageDrawable.java
AnimatedImageDrawable.cpp:
- new Drawable that handles animated images

Canvas.h, SkiaCanvas.h/.cpp
- New virtual method and implementation for drawing SkAnimatedImages

RecordingCanvas.h/.cpp
- Stub implementation of drawing SkAnimatedImages

ImageDecoder.h/cpp
- Allow code sharing with AnimatedImageDrawable.cpp
  - postProcess
  - access the ImageDecoder struct

Depends on https://skia-review.googlesource.com/c/skia/+/94660 in Skia.

Change-Id: Ie2ec98d9c52deda4d439c6ef8e5dea2861bb93a3