Fix bug #5332081 TextLayoutCache needs to be able to have more cache hits

- makes TextLayoutCache not carring about start/count. Basically he will cache the result for
the full string and gives back the "chunk" corresponding to start/count
- changed the TextLayoutCacheValue API to take start/count parameters
- added the Harfbuzz LogClusters in TextLayoutCacheValue as it is needed for extracting the start/count "chunk"
- fix potential issue of cache key leaking

Change-Id: I9276f9bec744e8de36349acfba8429f7c6f83394