[ical4j 4.x] Update "androidify" logic #225

Key points (as discussed in call):

• don't use data from ical4j timezone DB or VTIMEZONES if TZID is known in system
• otherwise (TZID not known to system), convert to correct timestamp + system default timezone
• move "androidify" code to where it belongs / make clear it's a mapping task
• also move storageTzId to where it belongs