Forces a back region for sandbox even if gesture nav is disabled.

When gesture nav is disabled, this config value returns 0,
preventing the back gesture from being recognized.
Probably we won't show the sandbox in that case, but it'd
be good to have a reasonable fallback just in case.

Bug: 186715828
Test: Manual with gesture nav enabled and disabled
Change-Id: Ic5e03bede80521670f81e6ac7b02f364259b9f96