[iOS] Pass NSTimeInterval instead of microseconds to Vsync tracing (#186190)

Fixes an issue in `FlutterVSyncClient` where platform vsync timestamps
were reported incorrectly to the timeline.

In https://github.com/flutter/flutter/pull/185918 I introduced the Obj-C
tracing API, which originally used microseconds as an int64_t. During
review this was updated to use NSTimeInterval to be more idiomatic and
use a clear type in place of int64_t for the timestamp. The tests were
updated but the callsite was not.

Authoring a test that would differentiate microseconds from fractional
seconds (NSTimeInterval) is infeasible in the current state without
significant refactoring, however, that refactoring is coming in followup
patch:
https://github.com/flutter/flutter/pull/186166

That patch could be landed as an alternative as it also contains the
fix.

Issue: https://github.com/flutter/flutter/issues/112232

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