According to hackbod on Stack Overflow, we're supposed to block in the
surfaceCreated callback until we've actually drawn into the surface:

http://stackoverflow.com/questions/8772862/surfaceview-flashes-black-on-load/8888108#8888108

This patch cleans up the public Java interface to the Flutter engine. The code
that we intend to be re-usable is now in the io.flutter.view package. The two
public classes are:

 * FlutterMain, which controls initialization of the system, and
 * FlutterView, which is an Android view that hosts a Flutter app.

We'll need to interate on these interface over time, but at least these names
are more reasonable.

We now use std::unique_ptr more consistently.

After this patch, we can execute Dart code and draw the result via MGL. We
still need to plumb in services.

This patch prepares us to add support for Mozart.

This CL separates the view-specific state from the process-global state. The
view-specific state now lives on Instance rather than Shell.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1187503003.

R=chinmaygarde@google.com

Review URL: https://codereview.chromium.org/1178773002.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1169983005.