[roll] Roll fuchsia [feedback] Persist system times on separate thread

This way, if the filesystem gets stuck Feedback has a better chance of
still being able to generate a snapshot.

Tested by temporarily inserting blocking code into SystemTimeTracker:

```
zx::nanosleep(zx::time::infinite());
```

and confirmed that system_time.json stopped receiving updates. Without
this change, `fx shell crasher` became stuck, but with this change a
snapshot was still generated @ 927904aace0be31e.

Original-Original-Fixed: 515487571
Test: `fx test //src/developer/forensics`
Original-Original-Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1658717
Original-Original-Revision: 931ac7bf8967b304d829eb63956b76838c2f75ce
GitOrigin-RevId: c946694a49f80dab146c02e92708e5ddf3f717d5
Change-Id: I6e58ecba8a4c96721b10951204fa6c55951587b4
Reviewed-on: https://fuchsia-review.googlesource.com/c/integration/+/1666755
Cr-Commit-Position: refs/heads/main@{#195182}