Use consistent terminology when referencing administration themes in the UI (not "admin theme")

Applied this Patch. It is a minor change and looks great.

So this is not the only reference to "admin theme" in our UI text. Most instances in the codebase are in assertions or code comments, but there are others in the UI:

[mandelbrot:drupal | Tue 15:58:07] $ grep -ri "admin theme" * | grep -v "//" | grep -vi "tests" | grep -v "*"
core/lib/Drupal/Core/Extension/ThemeInstaller.php:        throw new \InvalidArgumentException("The current admin theme $key cannot be uninstalled.");
core/modules/node/config/schema/node.schema.yml:      label: 'Use admin theme when editing or creating content'
core/modules/system/src/Controller/SystemController.php:        $theme->notes[] = $this->t('admin theme');
core/themes/seven/seven.info.yml:# 1. Copy the whole of Seven and rename it as your own admin theme. You

There are far more references to "administration theme", so I think this change is correct. Let's fix any others as well, though. We do not need to change test assertion messages or code comments, but we should fix any user-facing strings in the same issue.

Thanks!

Adding a new patch after fixing the points in #5 comment.

Not sure but i have find "admin theme" in whole codebase and found this in couple of other files as well (excluding comments n all). So Are we going to change these also ?

Here are some of the place where you can find the text that is i guess user facing.

core/modules/block/src/Tests/BlockUiTest.php
core/modules/menu_link_content/src/Tests/MenuLinkContentTranslationUITest.php
core/modules/node/src/Tests/NodeTranslationUITest.php
core/modules/system/src/Tests/System/MainContentFallbackTest.php
core/modules/system/src/Tests/System/ThemeTest.php
core/profiles/standard/tests/src/Functional/StandardTest.php
core/tests/Drupal/KernelTests/Core/Theme/ThemeInstallerTest.php

@Manjit.Singh In comment #5 it is mentioned that we don't need to change in test assertion messages or code comments.Applied patch working fine and i think conditions are also fulfilled.

This is a string change so to make it into 8.2.x needs to get in before RC.

Committed e94c581 and pushed to 8.2.x. Thanks!