Use a separate thread to decode AnimatedImageDrawable

Bug: 63908092
Test: Manual: Ie18811ba29a1db163aca08472b04ae185e9344f0

Depends on https://skia-review.googlesource.com/#/c/skia/+/101544. That
change removes the Skia class's time checks, and leaving it up to the
client to keep track of the time. In this case, the client wants to keep
track of the time because it only wants to update while it is being
drawn. If it goes off screen (for example), it will just resume where it
left off when it returns on screen. This allows for smooth animations.

If an AnimatedImageDrawable is being drawn to a SkiaRecordingCanvas,
decode on the new (lazily-created) AnimatedImageThread.

When running, always decode one frame ahead on the AnimatedImageThread
so that it will be ready when it is time to display.

During prepareTree, update the time and check whether there is a new
frame ready to draw or the next frame needs to be decoded. In either
case, return true. The next frame to be decoded will be triggered by
onDraw.

Change-Id: If447976e9df417060a950f658dbca9cf7980dd02