Guide behavior changes towards CompatChanges.

Each SDK level often has dozens of different behavior changes, which
can be difficult for large app developers to adjust to during preview
or beta releases. For this reason, android.app.compat.CompatChanges
was introduced as a new best-practice for adding behavior changes.

During a preview or beta release, developers can temporarily opt-out
of each individual change to aid debugging. This opt-out is only
available during preview of beta releases, and cannot be adjusted on
finalized builds.

Bug: 169879376
Test: atest error_prone_android_framework_test
Change-Id: Ib3b2e2139e084b0fa1bcbb5e89dd55e7ca4bfa00