pw_persistent_memory: reset() -> invalidate()

Renames reset() to Invalidate(), as it better expresses that the
resulting object will be considered invalid after calling.

Change-Id: I288ac1118b3af80f84a260ee979b81b684a43b9b
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/41341
Commit-Queue: Armando Montanez <amontanez@google.com>
Pigweed-Auto-Submit: Armando Montanez <amontanez@google.com>
Reviewed-by: Wyatt Hepler <hepler@google.com>