[iOS] Add unit tests for Default Browser status share

Adds some basic unit test coverage for the code path that shares the
default browser status to shared user defaults.

To make the attribution logic more testable, also tweaks how the
timestamp is generated so it uses base::Time::Now, which is overridable,
instead of [NSDate date].

Bug: 437153630
Change-Id: I07e0de23e1fb224beede2fc186ef3eef1a1353e5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6830310
Auto-Submit: Guillaume Jenkins <gujen@google.com>
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1498731}