Add StateTracker.

StateTracker is a special condition tracker that's based on a state atom.
State atoms are annotated in atoms.proto.

The rules for StateTracker:
 1. must not have "stop". must have "dimension"
 2. must be based on a state atom.
 3. it must have the all primary fields and the exclusive state field in its dimension.

 For example UidProcessStateTracker, will have output dimension {uid, state}.

Test: unit tests added.
Change-Id: I6b77e58e9fabe61f7326daf929577d8b2cfbf27b