tombstoned: Rename proto tombstone before text tombstone.

Because the log message "Tombstone written to: $NAME" is written
before renaming the proto tombstone, it is possible that a tool that
automatically downloads new proto tombstones by watching for this
log message will fail to download the tombstone. For the tool that
I am developing, this occurs < 0.1% of the time. Fix the issue by
renaming the proto tombstone first.

Bug: 328531087
Change-Id: I5b751585f2fce897ca7eb76c0466a0c33a93d851