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.

Don't claim to be a text editor

http://developer.android.com/reference/android/view/View.html#onCheckIsTextEditor()
says that we should return true if the "primary purpose" of
the view is text editing.  The keyboard still seems to work
fine if we return false here (default implementation) so
I'm removing this for now.

Fixes https://github.com/domokit/sky_engine/issues/116

@abarth

Fixes issue in sprite

Kill sky_server by port instead of pid

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.

Add PaintingContext which has-a sky.Canvas instead of is-a sky.Canvas

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.

sky_snapshot should work with absolute paths

Instead of appending the first path component, we should use it to initialize
`result`. That makes this code work with absolute paths.

Add a minimum sdk version for sky_engine

glFlush after inserting sync point

We could update all our other versions as well if we wanted
but this the only package in sky which uses sdk_ext

@abarth

RenderBox.hitTest should do the bounds check

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

Stop using workbench/packages for sky_snapshot

Instead we use the packages directory which is at the same
level as the BUILD.gn file.  That's a bit of a broken assumption
but it happens to work for all our skyx uses for now.

@abarth

@abarth @collinjackson

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