Fixes #6283

part of https://github.com/flutter/flutter/issues/5789

* make flutter run work with a pre-built apk

* refactor to remove the buildDir param

* iOS tools cleanup

1) Fix `flutter install` on both device and simulator to refer to the actual
   bundle and not just the .generated folder
2) Fix `flutter run` on device to actually run vs just installing

Still TODO:
1) Discovered that isAppInstalled on iOS simulator always reports true,
   meaning it'll never actually try to install the app.

Fixes #3947
Fixes #1823

Artifacts from local engine builds will be found based on the --local-engine flag

Host tools can be found in the artifact cache directory for the host platform.
If a developer wants to use a local engine build instead, then provide an
--engine-build flag that selects the specific engine build variant.

* rework flutter run

* fix npe with --debug-port

* connect to obs and exit when that conneciton closes

* update todos

All artifacts for iOS are universal.

Instead, require an AndroidManifest.xml and always build an APK.

Fixes #2517

flutter start's method of finding devices to run the app on is not suitable for flutter drive.

This commit also refactors several tool services to allow mocking in unit tests.

Use the project name to initialize the bundle name and identifier

remove device type specific checks

1) Moved basic utility code into base/ directory to make it clear which code
   doesn't depend on Flutter-specific knowldge.
2) Move the CommandRunner subclasses into a runner/ directory because these
   aren't commands themselves.

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

Instead of calling through `pub` to invoke build, this patch refactors the
build command so that it can be called directly.

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.