fastbootd: Disallow certain operations during snapshot updates.

When a snapshot is applied or is merging, requests to erase or flash
userdata, metadata, or misc must be protected. In addition, the
set_active command must be restricted when a merge is in progress.

In addition, introduce a "snapshot-update merge" command for assisting
with erase requests when a merge is in progress. As in recovery, this
will force a merge to complete.

Bug: 139154945
Test: apply update
      fastboot erase userdata
      fastboot erase metadata
      fastboot erase misc
      fastboot set_active

Change-Id: I152446464335c62c39ffb4cc6366be9de19eac30