Misc UI and logic fixes, and support for background sync.

* Moved preferences fragment into main activity. (Now in a view pager.)
* Better notifications:
  * Added string plurals and more detailed descriptions (e.g. "X songs
    downloaded, Y deleted")
  * No longer show error when sync cancelled by user
  * Click on finished notification launches app and dismisses
    notification
  * Fixed race condition that sometimes caused the in-progress
    notification to get stuck.
* Support for periodic sync in background