Use NAPlaybackIndicatorView itself as a pod

Also:

* Exclude the files from the Xcode project to avoid "duplicate symbol"
  error.
* Remove the link to QuartzCore framework in Xcode project since
  CocoaPods prepares it by the `s.frameworks = 'QuartzCore'` in the
  podspec.
Mark NAPlaybackIndicatorContentView.h as a private header