Move MimeMagic code to libs/mime/MimeAnalyzer.php

Move MimeMagic code to libs/mime/MimeAnalyzer.php

* The later resides in /libs with related files.
* Explose MimeAnalyzer as a service.
* Keep MimeMagic::singleton() as a b/c alias.
* MimeMagic::applyDefaultConfig() will bootstrap the service
  with all of the old config, extension hook handler, and
  detector command shell-out behavior.

Change-Id: Ie2695a52e7a3bcfda9f7fa83659a9ff31b372bc3