Issue #1798872 by vijaycs85, leschekfm, pcambra, andreiashu: Convert admin_theme() to CMI.

core/modules/block/block.admin.inc

...lock/lib/Drupal/block/Tests/NewDefaultThemeBlocksTest.php

...es/node/lib/Drupal/node/Tests/NodeBlockFunctionalTest.php

.../shortcut/lib/Drupal/shortcut/Tests/ShortcutLinksTest.php

core/modules/system/config/system.site.yml

...modules/system/lib/Drupal/system/Tests/Batch/PageTest.php

...odules/system/lib/Drupal/system/Tests/Menu/RouterTest.php

core/modules/system/system.admin.inc

core/modules/system/system.install

core/modules/system/system.module

core/modules/system/tests/modules/menu_test/menu_test.module

.../modules/taxonomy/lib/Drupal/taxonomy/Tests/ThemeTest.php

core/profiles/standard/standard.install