[iOS] Fix bugs in UTE metrics

OS version change should only be logged if an OS version is stored for a
previous session. Additionally, having an unknown amount of available
storage should not be logged as low storage.

Bug: 985784
Change-Id: Ie23eb993b1868e3cfef0360ceecdc594ff3f83fb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1710504
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679390}