Enable Back during setup wizard

Aside from enabling the system back button, updated the result codes
to distinguish back (RESULT_CANCEL) from skip (RESULT_SKIP).

This CL should have been uploaded along with http://ag/284254,
which has already been merged.

Bug: 6477759
Bug: 8409375
Change-Id: I031acda1d675ee79a3166a51073113093211709d