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.

remove the device factory methods as well as the device cache

rename to printTrace(), printStatus(), and printError()

remove device type specific checks

Sometimes "adb devices" prints a header. We should skip over the header
instead of complaining that we don't recognize it.

Fixes #1293

Catch the error and log a warning.

Fixes #1050

This is yet another hack on top of adb log handling.
This is fragile, but w/o this if you device time is out
of sync, you can't trace.

@abarth

update the min api level in the generated android manifest; use constants for the android version name

Fixes #858

@chinmaygarde

iOS takes a different codepath and will need more work.

@chinmaygarde

@chinmaygarde

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.

Now you can supply a route argument to flutter start:

  $ flutter start --route /foo

which will start the app at /foo instead of /.

Fixes https://github.com/flutter/flutter.github.io/issues/101