traced_perf: record "expired" (skipped) samples in the trace

Reader/unwinder previously dropped the samples in the kSkip state, which
arises when the proc-fd lookup times out (whether due to process no
longer existing, or the signal handlers on android being deferred for
too long).

Keeping the registers/stack while the expired entries are being enqueued
for logging (imo it doesn't matter how eagerly that memory is freed).

Not doing much with this information at the moment, but it's worth
including in the trace, for debugging or config tuning purposes.

+ wrap unwind_error in a oneof to be explicit about set-unset fields.

Bug: 144281346
Change-Id: I451802ba40b6b3725aa317831ead174c7582a91d