Date format improvements for new Japan era handling

Testing has revealed a couple of areas in which date formatting improvements are needed to handle issues related to the new 2019 Japanese calendar era:

1. Improvements in loading of intervalFormat data for skeletons with era (G+), used to format date ranges across era boundaries:

• DateIntervalInfo internal function validateAndProcessPatternLetter, add case for 'G'

• DateIntervalFormat::setSeparateDateTimePtn, at the end needs to call setIntervalPattern(UCAL_ERA,...)

2. SimpleDateFormat::applyPattern needs to update whether Gannen year numbering is used based on the new pattern being applied.

I already have code that implements all of this in ICU4C at least, will file PR soon.