fastbootd: Remove all scratch partitions on update-super.

This ensures that the dynamic "scratch" partition is removed when doing
a flashall operation.

If "scratch" is on /data, disable the partition. Add IsImageDisabled to
ImageManager so EnsureScratchMapped can skip mapping it. Also, fix
"scratch" not getting unmapped if on /data.

Bug: 205987817
Test: adb remount
      adb sync
      adb reboot fastboot
      fastboot flashall --skip-reboot # no errors
Test: adb-remount-test.sh
Change-Id: I4b9702e1dac15fb663635506fb50a8274e1e10d1