Refactor SearchableInfo.

- Removes the mSearchable field which was only
  for communication between the constructor and
  getActivityMetaData().
- Removes the badge and query rewriting fields,
  since their values can be efficiently computed
  on the fly.
- Makes all the other public fields private and adds getters
  for them.
- Makes all fields final, except mActionKeys.
- Removes the DBG_INHIBIT_SUGGESTIONS_CONSTANT.
  I don't see why we would every want that, and it
  complicated making the fields final.
- Makes all fields in ActionKeyInfo final.
- Makes all fields in ActionKeyInfo private and adds getters.
- Removes the use of ActioKeyInfo.mKeyCode for failure
  signalling. Uses IllegalArgumentException instead.
- Replaces the ad hoc linked list for looking up
  action keys by a HashMap. This is needed to
  make the fields in ActionkeyInfo final, and also avoids O(N)
  lookup in the (unlikely) case that an activity
  has lots of action keys.
- Don't throw exceptions when reading searchable
  meta-data, since that could crash SearchManagerService.
- Adds debug logging.