Helper function to mount snapshot devices in recovery

In recovery the user has the option to mount /system, but when a device
is updating with Virtual A/B, the _b partition may be a snapshot.
This patch introduces a helper function that satisfies the dependency of
setting up the snapshot devices required to mount /system.

Bug: 139157327
Test: manual /system partition mount on VAB device during OTA
Change-Id: I7337bdd38d7016d12d3ee42be1c7893b10e9116d
Signed-off-by: Alessio Balsini <balsini@google.com>