Remove unnecessary braces in PlaybackService case block

The other blocks in this switch don't have braces, and they would look a
little silly when I add an extra case label.
Support next/previous media keys

The "seek" buttons in my car send "next" and "previous" media key codes,
rather than "fast forward" and "rewind", so update the playback service
to recognize them.

fixes #386
fixes #432
Show a toast when an unknown media key is pressed

This is just an idea.  If people want more media keys supported, they
will be able to use this toast to report exactly what key codes their
hardware is generating.

If you don't want to include this, just let me know and I can remove it
from the pull request.
Make seek time configurable

A 30-second seek time is way to long for me.  Make the seek duration a
configurable preference.
Merge branch 'seek' of git://github.com/dreiss/AntennaPod into dreiss-seek
Only report unknown keycode if PlaybackService is playing