Revert "Disable SliceBackupHelper on devices with FEATURE_SLICES_DISABLED"

This reverts commit 38eb00d6.

Reason for revert: b/277914315 (my change from tm-wear-kr1-dev breaks master on merge, need to update a test in master which isn't in T before merging)

Change-Id: I3d57767e82ff8cd73255e0973fe8f03cb9413dc0