More finishing-up of cluster implementation

Implemented the nextClusterForward attribute (didn't do anything
before).
Fleshed out javadoc links.
Clusters now share sorting with normal focus.
Added sound on cluster navigation.

Bug: 34362469
Bug: 34363357
Bug: 34362653
Bug: 34361907
Bug: 34363159
Test: Added CTS to verify clusters behave the same as normal focus
      wrt user-specified ordering. Tested manually that cluster-
      nav generates sound.

Change-Id: I810acc058d9c1fad4a085d9b7f267e9ec14b33e7