snapuserd: API to query snapshot and merge status

Add new API to query the snapshot and merge status.
This will be used by libsnapshot.

Bug: 193863443
Test: Full OTA on CF
Signed-off-by: Akilesh Kailash <akailash@google.com>
Change-Id: I86cffff6a979e2e2bf1d8d1a1770e209eeb4a47d