Eleven: Make fastforward/rewind more reliable

Part of the logic was in the ui to control the fastforward/rewind
when there is no need, and the asynchronisity of it was causing problems.
This moves the logic for seeking all into the service so that the ui only
tells the service that it should rewind/fastforward, and by how much

https://cyanogen.atlassian.net/browse/MUSIC-167

Change-Id: Id7623a1a755c7b1346874bdcf64126123eafc7d1