Merge "Make docs clear that you shouldn't change an existing onUpgrade step."...

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

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



Change-Id: I17dfc9ea8aa9f88e03ec7c44168e3c0afd6fc1e7
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>