Rename to HsumDefaultConfigOverlay

As a workaround to a caching bug, we rename
the old HsumConfigOverlay to HsumDefaultConfigOverlay.

Bug: 357828774
Test: manual (flash build from a week ago, flash this build without
      wiping, make sure the system boots)
Flag: EXEMPT refactor (and bug fix)
Change-Id: Iac7c07c68dfdc4e00afd3f89d4c26ca10adbaa3e