Merge "Make docs clear that you shouldn't change an existing onUpgrade step." into udc-dev am: 5c876772 am: 712db9d7

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/22103823



Change-Id: I56eea30111e25e40919222fbeee8417ebfd0c23f
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>