libsnapshot: Add a compression bit to SnapshotStatus.

Using ro.virtual_ab.compression.enabled does not work in first-stage
init, since properties aren't available. Instead, attach a compression
bit to SnapshotStatus. Although there are no plans to have per-snapshot
compression toggles, this route avoids creating a new state or indicator
file elsewhere. And SnapshotStatus is available in every place that
cares about compression.

Bug: N/A
Test: apply OTA with VABC
Change-Id: I51deb5693a08417ea7a2c524e240191748907123