[KMediaSession] Only send MRPIS2 positionChanged signal on seek

Stop spamming the DBus with positionChanged signals if playback is
progressing at the nominal playback rate.  This brings the
implementation in line with the MPRIS2 player spec.  The previous
implementation was causing android battery drain issues with KDEConnect
constantly updating the position (and player details) on the notification
widget.
This implementation will still send the current playback position every
10 seconds to ensure that clients that don't implement the standard
properly or did not receive previous messages, will still get regularly
updated.
This also fixes correct signaling of playback rate changes over DBus.