\Drupal calls should be avoided in classes, use dependency injection instead

Please review https://git.drupalcode.org/issue/content_snippets-3436387

@chaitanyadessai thanks for this! It's been tweaked a bit and committed on the 2.1 release. Feel free to review the changes I made -- the `use` statements should be alphabetized and trimmed, and I like a slightly less complex pattern for the constructer.