libsnapshot: Check if OTA update in progress during reboot

If any of the read-only partitions are mounted off dm-user
then certainly update is in-progress.

Bug: 308900853
Test: OTA on Pixel, reboot during OTA.
Change-Id: I36121e1d99ec7c1f1110a65fc67996190875af18
Signed-off-by: Akilesh Kailash <akailash@google.com>