More battery history improvements.

- Better batching of history items.  Fixed problems where empty
  entries would be created because state toggles got lost.
- The string pool is now a HistoryTag pool, containing both a string
  and uid; now an entry only requires 16 bits in the history data.
- Acquiring the first wake lock also now includes a HistoryTag
  identifying who did the aquisition.
- Cleaned up printing of signal strengths and cell radio types.
- There was code that tried to allow you to add new history entries
  while iterating the history...  but these should never happen
  together, so turned that into a failure...  and fixed an issue
  where we could leave the battery stats in a state where it
  thinks it is continually iterating.

Change-Id: I1afa57ee2d66b186932c502dbdd633cdd4aed353