R1:25aebab655fd

Make misc get*ApplicationClass[Name]() calls return ::class constant instead of string

Summary:
`::class` is available since PHP 5.5 (5.5 is a minimal requirement by Phorge): https://www.php.net/manual/en/language.oop5.basic.php#language.oop5.basic.class.class
It makes finding code using IDEs easier; see discussion in D25500.

Thus replace all string return values with returning the `::class` constant instead, with one exception: 'PhabricatorSettingApplication' in `PhabricatorUserPreferencesSearchEngine.