Remove JapaneseCalendar.CURRENT_ERA from Android public SDK

JapaneseCalendar.CURRENT_ERA is final static int, and the developer
may mis-use and could have wrong expectation on this API, since
1. New emperor in Japan could result in a new era, but old Android device
   still provides an outdated "current" era.
2. Developer may expect its value depending on system time. However,
   given it's a static final int,
   Android could intiailize the value at boot with an incorrect time,
   and thus results in incorrect era. Thus, on Android, The value is
   always hard-coded.

Bug: 131893534
Test: make
Change-Id: I16ea841975dd8e913409c884123003f842c55bb8