libsnapshot: Add a compression bit to SnapshotUpdateStatus.

This adds a compression bit to SnapshotUpdateStatus. This is so init can
tell whether it needs to launch snapuserd, without reading the
individual state of each snapshot, since this state is global.

Bug: 173476209
Test: manual test
Change-Id: I8c3bbfb0d184f70e661e7b043afc37e335d1e187
3 files changed