ICU-20467 get XLocaleMatcher ready for drop-in

ICU-20467 get XLocaleMatcher ready for drop-in

Get XLocaleMatcher ready for replacing the LocaleMatcher code.
More simplifications beyond ICU-20330 PR #409, smaller data, some more optimizations.
New API ready to be moved over.

- less work for region partitions distance lookup:
  - encode each array of single-character partition strings as one string
  - look up each desired partition only once, not for each (desired, supported) pair
  - look up the * fallback region distance only for the first mismatch, not for each non-matching pair
- skip region distance lookup if minRegionDistance>=remainingThreshold
- locale distance table: remove subtables that contain only *-* with default script/region distance
- mark intermediate subtag matches via last-character bit 7, not also with a match value
- likely subtags data: prune trailing *-only levels, and skip *-only script levels; likely subtags perf test
- likely subtags: skip_script=1; LSR.indexForRegion(ill-formed)=0 not negative
- likely subtags small optimization: array lookup for first letter of language subtag
- defaultDemotionPerDesiredLocale=distance(en, en-GB)
- favor=script: still reject a script mismatch
- if an explicit default locale is given, prefer that (by LSR), not the first supported locale
- XLocaleMatcher.Builder: copy supported locales into a List not a Set to preserve input indexes; duplicates are harmless
- match by LSR only, not exact locale match; results consistent with no fastpath, simpler, sometimes a little slower
- internal getBestMatch() returns just the suppIndex
- store the best desired locale & index in an LSR iterator
- make an LSR from Locale without ULocale detour
- adjust the XLocaleMatcher API as proposed; remove unused internal methods; clean up LocalePriorityList docs