libsnapshot: Pause snapshot merge during shutdown

If the device is mounted off snapshots, initiate the snapshot-merge
pause.

Additionally, try to umount the partitions which are mounted off snapshots.

Bug: 386142969
Test: Reboot device when snapshot merge is in progress and verify merge
threads are paused
Change-Id: Ide9e33157432e3a8d4e6ba7d4d414270505b8941
Signed-off-by: Akilesh Kailash <akailash@google.com>