snapshotctl: Add a cancel command.

This is for host-driven testing. update_engine_client --cancel doesn't
see an OTA in progress if it's already been staged, so we need a lower
level command to remove snapshots.

Bug: 352539864
Test: atest RecoveryOtaTest
Change-Id: I4c6c0923b0369af70a2ab941a5c60e471a46f9b3