Updates to game sound API

Add a compositing step to the lifecycle

Now we have the ability to draw multiple PictureLayers. We still squash all the
pictures together into a single SkPicture for simplicity. In the future, we'll
submit them to C++ separately and composite them on the GPU thread.

Adds CicularProgressIndicator and LinearProgressIndicator

Setting a ProgressIndicator's value to null (the default) makes it an "indeterminate progress" or activity indicator.

The indeterminate animations for both kinds of progress bars are essentially the same and wrong vis the Material Design spec, http://www.google.com/design/spec/components/progress-activity.html. I'll improve conformity with the visual design in a future CL.

Setting a ProgressIndicator's value to null (the default) makes it an "indeterminate progress" or activity indicator.

The indeterminate animations for both kinds of progress bars are essentially the same and wrong vis the Material Design spec, http://www.google.com/design/spec/components/progress-activity.html. I'll improve conformity with the visual design in a future CL.

Expose longpress and showpress in Listener

Removes the ability to jump to different positions in sound effects (this isn't supported on Android)

Currently we have a single PictureLayer that everyone draws into. A future
patch will teach the system to use multiple PictureLayers.

Adds rough sound support for sprites

Rename createsNewDisplayList to requiresCompositing

This patch prepares for introducing the compositing system by repurposing the
relevant parts of the createsNewDisplayList system. This patch also removes the
no-longer-relevant parts of that system.

Fixes issue in sprite

We seem to have trouble killing sky_server by its pid. Instead, lets try
killing it by looking up which process is listening on the sky_server port.

Fix common asserts in animation API.

We'll need this for compositing because we need to switch out the sky.Canvas
when we switch compositing layers.

1. We would assert if you tried to start an animation from within an
animation status callback. This is a common pattern, so I fixed this
assert (in Ticker._tick).
2. We would assert for animations with duration under a millisecond.
Fixed.

Also removed the workarounds assocated with #1.

Previously we were doing the bounds testing for hit tests in the parent, but
that doesn't work if the child paints at a location other than 0.0, 0.0. Now we
do the bounds check in the child. This also simplifies Scaffold's hit testing.

Fixes #558

Add support for charts to fitness app

Adds ensureWidgetIsVisible() function to scrollable.dart

Maximum bounds for the mimic in the overlay can now be specified.

Adds API docs to particle systems and removes redundant property

Set the scrollOffset of a widget's Scrollable ancestor so that the
widget is centered within the scrollable. A future CL will add
support for specifying exactly where the widget appears. The scroll
can be animated by specifying the animation: parameter.

Changed the duration Scrollable.scrollTo() parameter from a Duration
to an AnimationPerformance so that one can configure all aspects of
the animation. The caller may also listen to the animation to schedule
other work while it updates or when its status changes.
complete