Deprecate internal Calendar methods.

These methods suffer from a potential integer overflow that can't be
fixed without changing their signature as described at
http://bugs.icu-project.org/trac/ticket/11632.

Since they could be overridden, we need to deprecate them for at least
one release before changing/removing them.

Bug: 30300853
Test: m checkbuild && m offline-sdk-referenceonly-docs
Change-Id: I07e14de697c1551b8d99fbd5af2a56a498226e66