Move shelf config setting into SystemUI

It doesn't need to be visible to the rest of the framework.

Also add a system setting so adb can override the default before for demos:
adb shell settings put system demo_app_shelf 0  # Zero forces it off.
adb shell settings put system demo_app_shelf 1  # Non-zero forces it on.

Bug: 22332617
Change-Id: I75b228a24b307447a7e6df11ba0d024b88759beb