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
- add Harfbuzz LogClusters in TextLayoutCacheValue as it is needed for extracting the start/count "chunk"

Change-Id: I4b38a4442428606de9a093303bbbe98181e1f89c