power: Keep file scheme when not using content URI

 * Settings.Global.CHARGING_STARTED_SOUND and
   Settings.Global.WIRELESS_CHARGING_STARTED_SOUND are
   in old format and do not use content URI.

   Therefore, we should preserve AOSP logic and prefix sound URI path
   with "file://" or there will be warnings like this:

     W MediaPlayer: java.io.FileNotFoundException: No content provider: /system/product/media/audio/ui/WirelessChargingStarted.ogg

   If users have changed charging sound at some point, then it is fine,
   because we have switched to content URI.

Change-Id: I63bdbe1003390310facbb50175e961129ecb3118