libsnapshot: Clarify the meaning of "compression.enabled".

The compression.enabled property only refers to legacy snapuserd.
However, everywhere it's used, we almost always mean "using the new COW
format with snapuserd, whether compression is enabled or not".

This patch clarifies things by renaming "compression_enabled" fields to
"using_snapuserd".

This also simplifies the feature detection code quite a bit, by removing
some redundant checks and clarifying which cases are legacy vs userspace
snapshots.

Bug: 208944665
Test: vts_libsnapshot_test
      apply OTA, snapshotctl dump
Change-Id: Ie617ed3f92eefb8dff0a5cf13e5a2caa47b9e8e0