libsnapshot: Integrate userspace snapshots APIs

dm-user block device will be the snapshot device; thus, no
more explicit call to MapSnapshot(). Additionally, block device
name for dm-user will be the snapshot name so that mount works
seamlessly.

API's to query the snapshot status, merge progress has been
integrated. Since daemon requires base device for merge, we pass
additional parameter during initialization.

Add a new virtual a/b property flag to enable/disable
user-snapshots feature. Propagate this flag to init layer
for first stage mount during boot process.

Some minor cleanup and renaming of variables.

Bug: 193863443
Test: 1: Full OTA on CF and pixel and verify the merge completion.
Tested merge-resume path by rebooting device during merge.
2: Incremental OTA on CF and pixel

Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I5088f40a55807946cd044b3987678ead3696d996