Separate the ICU API from the android-non-updatable API

Added 31/public/incompatibilities.txt to describe the incompatibilities
between version 30 of android-non-updatable and the current version.

Bug: 174344410
Test: lunch sdk-eng && m dist sdk
      Run above before and after this change to ensure that the
      runtime-i18n stubs jar and API specification file (but not the
      removed API specification file) are copied to the dist.
      Also, added a new method to the
      prebuilts/sdk/30/public/api/runtime-i18n.txt and run m checkapi
      to ensure that incompatible changes between the latest released
      API and the current API are detected properly.
Change-Id: I6ac4af8c6930ad7cfb9ddaee519778519fde7ad4
Merged-In: I6ac4af8c6930ad7cfb9ddaee519778519fde7ad4
3 files changed