Add ChangeId for app data isolation.

Currently we are using DeviceConfig to set the target SDK level as an
experiment to determine if this causes any significant problem for
existing apps. So for now we just report the change, but once that is
done we can switch to using this for targeting.

Test: Boot device, see "Compat change id reported: 143937733; UID
<various>; state: LOGGED".
Bug: 147266020

Change-Id: Ic7f9468fa5b246c0d8e52a1b321c87e72933ca76