This flag builds a local FLX file and pushes that to the device instead of
using an HTTP server.

This patch makes `flutter start` work without a clone of the engine git
repository. Making this work pulled a relatively large refactor of how the
commands interact with application packages and devices. Now commands that want
to interact with application packages or devices inherit from a common base
class that holds stores of those objects as members.

In production, the commands download and connect to devices based on the build
configuration stored on the FlutterCommandRunner. In testing, these fields are
used to mock out the real application package and devices.

Also fixes some minor bugs with iOS and Android interactions.

Also update tests to be compatible with the presence of iOS and add tests for list and install.

`dart bin/sky_tools.dart --debug --sky-src-path=/path/to/sky/src/ install` now works.