This consolidates all of the non-template .gitignore rules into the top level .gitignore, to ignore common things more broadly, with less maintenance needed for the .gitignore files. Does not touch the templates, so that they still produce needed .gitignores as part of flutter create.

* Add framework-side support for system text scale factor.

* Rolling engine to e3404b81a53ba3180c7623a6f2190ebb28518f30
  Additional changes rolled in with engine change:
    libtxt: implementation of GetRectsForRange that processes a line at a time - e3404b8
    Provide an entropy source to the Dart engine (#4161) - e1aa867
    libtxt: search for fallback fonts that can match emoji and CJK characters - 8061df1
    Roll skia to e4679fa06a. (#4157) - 267e7a8
    Update buildroot to 53fea9aebbcc39c6522731471a1a45960ee0685e (#4160) - 02ea7ae
    Revert engine Dart roll. (#4158) - 14aab33
    Add support for system text scale factor. (#4124) - b2a7f4b
    Include _http into sky_engine libraries for analyzer (#4154) - b930f10
    libtxt: Remove postprocess_line and improve tracking of X offsets - 86f95f0
    libtxt: remove redundant line_widths (#4152) - 14bf515
    Roll dart to ade37f931e90b0fdb8fe16d6bf6f089545da55b6 (#4151) - 6f1264f

Under android/ directory, ignore:
GeneratedPluginRegistrant.java

Under ios/ directory, ignore:
GeneratedPluginRegistrant.h
GeneratedPluginRegistrant.m
Pods/

* Roll android build tools to 25.0.3

* Roll android build tools to 25.0.3 in templates

This reverts commit 5ed2984e.

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

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

Gradle projects are evaluated in lexicographical order, and the plugin
projects are at the same level as the :app project, so if a plugin has
a name that comes before 'app' (like, for example, any name that starts
with a capital letter), the plugin project will be evaluated before
:app.

Since :app applies the Flutter Gradle plugin, which tries to
modify the dependencies of the plugin projects, we have a problem if the
plugin projects have already been evaluated. Adding
evaluationDependsOn(':app') to the plugin projects fixes this.

Updated example projects to the latest (plugin-enabled) Gradle build
files.

Also removed two unused imports in `pluginClass.java.tmpl`.

This reverts commit 8da4778e.

* Update hello_world to new gradle based build flow

* Update flutter_gallery to gradle based build

* Align hello_services with new gradle based build

* Update platform_services to new gradle based build

* Upgrade stock to gradle based build

* s/sky/flutter/ in Android templates

* update engine.version with a compatible engine version

* replace more SkyActivity references with FlutterActivity

This is needed to update an app's layout when a soft keyboard is shown

* making flutter run work with gradle

* locate android studio

* add test for settings

* review comments

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

Fixes #2517