fastboot: Implement helper commands for Virtual A/B.

This introduces two new commands to the fastboot protocol:

  - getvar snapshot-update-status - Return "none", "snapshotted", or
    "merging" depending on the current status set by the boot control
    HAL.
  - snapshot-update [cancel] - Cancel any pending snapshot-based updates
    via the boot control HAL. After this, the HAL should return
    MergeStatus::CANCELLED and "update-merge-status" should be "none".
    If no argument is specified, the snapshot-update-status is returned
    via an INFO response.

Bootloaders are expected to implement this in a manner consistent with
the boot control HAL.

Fastboot-based tooling should expect wipes of userdata to fail when
update-merge-status returns "merging". Thus, the force flag now cancel
any pending snapshots.

Bug: 139154945
Test: fastboot getvar snapshot-update-status
      fastboot snapshot-update cancel
      fastboot snapshot-update

Change-Id: Idc423fe7656b212e929e64eb0e6b85b453e0e8dc