init: umount dynamic partitions during shutdown

If the device is mounted off snapshots, try to unmount
dynamic partitions.

Bug: 386142969
Test: Reboot device mounted off snapshots and verify partitions
are unmounted
Change-Id: Idc8d65750f0d336d12e6dfe78f16dc911b0dea3c
Signed-off-by: Akilesh Kailash <akailash@google.com>