Use DM layer directly to manage private DM volumes

Abolish cryptfs_revert_ext_volume, handle in caller. This allows us to
use DeleteDeviceIfExists, avoiding a spurious error message.

Test: create private volume on Cuttlefish, eject, check logs
Bug: 147814592
Change-Id: I836d8bd11b29e32da0863aaa75144543bb9cab9c
3 files changed