revise accessibility methods in postflight miniDSL

* rename `remove_accessibility_access` to `disable_accessibility_access`
  to match corresponding method `enable_accessibility_access`
* move `TCC.db` and `.AccessibilityAPIEnabled` paths to `Cask::Locations`
* remove unneeded backslash from `TCC.db` path
* use full paths to `touch` and `sqlite3` utilities
* use `@command.run` instead of `system_command` which is intended as an
  external interface
* reverse conditional so enable/disable logic is consistent
* `sudo` is needed when creating `.AccessibilityAPIEnabled`
* instead of silent fail, warn that access cannot be safely disabled on
  Mountain Lion and earlier (it could, but might affect other apps)
* quoting/whitespace