libsnapshot: Fix checks for compression to work with new snapuserd.

Most checks for compression are used as a gate to decide whether dm-user
is enabled. However, dm-user can now be enabled without compression, so
these checks have to be audited and adjusted.

Once the "old" compression path is removed, these checks will become
simpler.

Bug: 208944665
Test: vts_libsnapshot_test -force_config vab
Change-Id: I281937e26427044f1ef2e1164d86d67e4170163a