Improve Android-changed documentation for removal of ZoneInfo

This CL adds documentation for previously undocumented Android
changes in AbstractCalendar, and makes the change comments
consistent.

sun.util.calendar.ZoneInfo appears to never have been available on Android.
Libcore commit 9c853c5b9ebbb0ef60a013ae10ee411d70dfa832 back in 2015 made
corresponding Android changes to the OpenJDK code that was ported, but
those changes were not documented at the time.

Bug: 35910877
Test: Treehugger build succeeds.

Change-Id: I4842885ebb5a0272f545cea459413155fb98e6dc
3 files changed