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

* Revert "Update Hello World to latest iOS project template (#8634)"

This reverts commit 6dc6ed4e.

* Revert "Update Gallery to latest iOS project template (#8637)"

This reverts commit 84c42aac.

* Revert "Update Stocks to latest iOS project template (#8640)"

This reverts commit 90774c18.

Gallery and stocks:
* Add 20x20 notification icons.
* Remove unused 76x76 (3x scale) icon.

Other examples: update JSON to Xcode 8.2.1 defaults

Fixes warnings for missing/unused icon assets in Xcode 8.2.x

Subclassing FlutterAppDelegate is not required for real-world apps, but
it's what 'flutter create' generates, and applies the default behaviours
that most real-world apps will want.

Almost all real-world apps will want a custom app delegate, and 'flutter
create' code-gens one by default. This brings the samples in line with
our templates and the most common use case.

As of Xcode 7, Apple recommends setting the bundle identifier in the
PRODUCT_BUNDLE_IDENTIFIER build variable and setting CFBundleIdentifier
to that variable in Info.plist.

As of Xcode 8, CLANG_WARN_INFINITE_RECURSION and CLANG_WARN_SUSPICIOUS_MOVE
are recommended settings.

We want to use the FlutterViewController in this new project template.

Fixes #4840

We now use a different approach for ios projects where the developer controls
the Xcode project file. This patch removes the old ".generated" approach in
favor of the new approach.