apply-update: Add option to boot device off snapshots

$apply_update --boot_snapshot

~20 seconds to boot off the snapshots

To revert back to original state:
`adb shell snapshotctl revert-snapshots`

Bug: 400723109
Test: apply_update --boot_snapshot
Change-Id: Ia71c93b606149f0f0824085b50a9dc9e76ac5d2a
Signed-off-by: Akilesh Kailash <akailash@google.com>