Enable plugin infra for both iOS and Android post-#9791.

* Minor grammar fixes.

* Update README.md

Trivial doc typo.

* Remove all initial cocoapod traces from flutter create except Podfile

* Make updateXcodeGeneratedProperties parameters named

* review notes

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This reverts commit 9f5fa31e.

Brings hello_world and flutter_gallery Android code in line with the
corresponding iOS code.

Also, I had a question about flutter_test matchers and our style guide
says that when I have a question I should update the docs so I did
that and then got a bit carried away.

See https://github.com/flutter/flutter/issues/9726; for some reason
the markdown in this file is getting misinterpreted.

Fixes #9757

Fixes https://github.com/flutter/flutter/issues/9342

* Revert "Revert "Don't try to resolve CocoaPods unless project has plugins" (#9719)"

This reverts commit f739e9e0.

* Remove channels integration test’s partial cocoapods artifacts

Fixes https://github.com/flutter/flutter/issues/7988

* Let printError print bold as well

* review notes

Fixes https://github.com/flutter/flutter/issues/8476

More detailed list of changes in this patch:

* Replaced the didTouch special logic with more generic logic that
  uses Activities instead. Now instead when you tap down the
  Scrollable calls `hold()` which begins a HoldScrollActivity which is
  a hybrid of DragStartDetails and IdleScrollActivity and can be
  canceled. When you let go, it gets canceled and that goes ballistic.

* Make DragGestureRecognizer more aggressive about grabbing pointers,
  otherwise a second pointer in a situation with competing horizontal
  and vertical recognizers always gets taken by the other one.

* Fixed the _GestureSemantics widget to call the "down" callbacks so
  that it follows the same pattern as "real" interactions.

* Added tests for the above.

* Added a hashCode to ScrollActivity.toString (and subclasses).

* Added a toString to ScrollDragController, and include it in
  DragScrollActivity's toString.

* s/coorindator/coordinator/

* Add a comment in DragStartDetails to distinguish it from the
  otherwise identical DragDownDetails, so we're not tempted to merge
  them.

* Don’t automatically pod setup if the user never did it

* fix/add test

* rename getters

* fix an issues with reloading when paused at a breakpoint

* update docs

Eliminates Gallery-specific hardcoding of app package and activity name
in the test.

Fixes breakage introduced with 1dc2d4eb
due to main activity class now being MainActivity.