dumpstate: Handle prev_boot_dump.trace specially

prev_boot_dump.trace is a special trace stored by the kernel of its last
boot events. This is specially useful when crashes are happening.

DD: go/persistent-tracing

Bug: 412895485
Test: See the the file is reported and not removed
Flag: NONE do nothing if the file does not exist
Change-Id: Ie5c8eb6f6584c1c71616db7ed2a3180d9b788ffd