fastbootd: Partition commands should update all metadata slots.

Without this, it is much more difficult to ensure that the "a" and "b"
slots are both bootable during development. We already update all
metadata slots for update-super, so we should here as well.

Bug: N/A
Test: fastboot flashall
      fastboot set_active other
      fastboot flashall
Change-Id: Ib661e35fa89171a68a0b1da195dc5ba0375d72e4