Changed rewind/fast forward button actions so holding will skip tracks

[KBerstene]

When in the Now Playing fragment, holding the previous/next button will enable an auto-repeating rewind/fast forward action until the button is released. However, holding the rewind/fast forward buttons will not do anything, it will just do a normal rewind/fast forward action when the press is released.

I have changed the actions so that the rewind/fast forward buttons are better "mirrors" of the previous/next buttons--that is, holding the rewind button will call the previous track action, just as holding the previous button will call the rewind action (and the same for next/FF). I didn't keep the auto-repeating functionality for this, since holding to skip tracks is odd and could be unexpected behavior. Instead, a simple long-press triggers the action, instead of the auto-repeat touch event.

This change also fully separates the rewind/fast forward actions from the next/previous actions. In the current version, the next/previous action would check to see if rewind/fast forward should be called. This can cause unexpected behavior. If I press the "next" button (in multiple contexts, whether in the app itself, on the widget, notification, or Auto controls), I would expect the next song to play, and not the song to simply fast forward. I understand that this check was in there due to rewind/fast forward buttons not showing up in different contexts - I am working on another commit for the home screen widget and for Android Auto. GlennGuy's excellent MediaStyle notifications are already behaving properly with rewind/fast forward button appearances, though that pull request has not been accepted yet. If for some reason it is not accepted, I can work on notifications as well.