Re-enable dreams: frameworks/base

Enable feature in config. Expose Dream in public api for unbundled apps.
Unhide package.  Add isDreaming() method to service.

Re-arrange the Dream api a bit.  (use onStart as hook for subclasses).
Coordinate properly with power manager.

Replace old dock mode (don't fire old intent).

Change-Id: I1318d20cc1613e5d862f2913f2fcdc9719302cf7
Bug: 6921930