These are required to upload to the store.

Fixes #1006

Instead of running the FLX directly, we need to run mojo:launcher and have it
run the FLX.

The IOS code is totally untested. Need a Mac to test it.

This provides a fast way to iterate on changes to a Flutter app that only
involve updates to Dart code and do not require a full build and install
of the FLX and APK

These classes now share more code and have feature parity.

Previously we'd always stop org.domokit.sky.shell. If an
AndroidManifest.xml exists, we'll use the activity specified in there
instead.

remove the device factory methods as well as the device cache

add device notifications for the simulator

This splits flx.build() into two methods, flx.build() and
flx.assemble().  builD() now does the following:
1) constructs the manifest map by reading the manifest from the
   file system
2) "compiles" the dart code into the snapshot file
3) Invokes assemble()

This allows external build toolchains to construct their own
manifest map (possibly using a different manifest syntax)
and create their own snapshot file

flutter start no longer depends on a pre-built SkyShell.apk. It builds a
new one, as long as an AndroidManifest.xml exists.

We rebuild the .apk every time either AndroidManifest.xml or
flutter.yaml changes.