New loading model

 Rename reload() to loadIfDirty()
 loadIfDirty is implemented by MediaSet and is final
 loadIfDirty is now blocking instead of async
 Subclasses must implement two protected methods, isDirtyLocked() and load()

 The change from async reload() to sync loadIfDirty() should be fine since
 all users of reload() were doing so from a background thread already, and the
 longest load() is PicasaAlbumSet which is still a fairly brisk 40ms or so

Change-Id: If5cc596a1c13e52e5f4efff1a144bd086d37cfb7
24 files changed