Going forward, Android support libraries are published on maven (instead of bundling them with the SDK). Many plugins depend on these. To avoid requiring plugin users to add the maven repository to their app this change adds the repository to the template for `flutter create`.

This also bumps the support-annotations dependency to 25.4.0 (which also requires the new maven repository).

* Roll android build tools to 25.0.3

* Roll android build tools to 25.0.3 in templates

This reverts commit 5ed2984e.

* Add flutter_view sample

* Removed platform_services files

* Addressed comments

* update README.md

* Addressed comments.

* Make new project template gradle-based for Android.

With this change, the 'new project' template uses the same gradle-based build for Android as the hello_services example. This has some implications on build performance, since we're now building a complete Android app instead of just combining a pre-compiled .dex with the Flutter assets.

The very first build is a little over 2x slower, since it needs to download gradle and build the build scripts before getting to the actual code. Subsequent builds with changes to the code are comparable to the old builds. Null builds are faster. Enabling the gradle daemon speeds up subsequent builds by around 5s.

* Move Flutter Gradle plugin to Flutter root.

* Update gradle example to support x86 in debug mode.

Changed the Flutter Gradle plugin a bit to better fit in with the
Android build.

Fixes #6136
Fixes #6864
Fixes #7539

The hello_services example can now be built both for Android and iOS.