Add annotation to atoms that represent a state change in atoms.proto

+ A state change atom can have one exclusive state field, and any
  number of primary key fields.

  When there is primary key in the atom, it means the state belongs to the primary key.
  For example,
  message UidProcessStateChanged {
    optional int32 uid = 1 [(stateFieldOption).option = PRIMARY];
    optional android.app.ProcessStateEnum state = 2 [(stateFieldOption).option = EXCLUSIVE];
  }

  When there is no primary key fields in the atom, the state is global.
  For example,
  message ScreenStateChanged {
     optional android.view.DisplayStateEnum state = 1 [(stateFieldOption).option = EXCLUSIVE];
  }

+ The annotation is consumed by stats_log_api_gen to generate a static map from the state
  atoms to its primary fields, and exclusive fields

+ stats_log.proto is splitted into 2 proto files, because statsd needs proto lite, and c++
  lite proto library cannot properly ignore the field options which requires full proto.

 This CL doesn't change any logic in the statsd yet. A separate CL will use the field option
 information to correctly track the state.

Test: added unit tests in stats_log_api_gen_test. and statsd_test pases.
Change-Id: I9e8a979fe81ba60efd4d854bb7087ce4b2b147ec