A common use case for members of the Flutter team is to have a dependency
override for the flutter package that points back into the engine src tree.
We can use that override to automatically detect the engine src path, which
makes the command line shorter.
This patch adds a couple print statements to explain why the first run of
`flutter start` takes a while. (We need to download the APK and install it on
the device.)
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.
