perf(focus-monitor): avoid triggering unnecessary change detections

[mleibman]

This is similar to #15077, but IMHO does it in much simpler way. I also fixed up the documentation, examples, and some call sites that incorrectly stated thatFocusMonitor emits the observable outside of NgZone.

Caretaker note: #15077 had to be reverted before (#15076) because there was an app that depended on the extra change detections. We may have to investigate further before we can merge this in again.

[andrewseguin]

Please rebase, thanks!

[andrewseguin]

Needs rebase (sorry, this is taking a while due to failures internally)

[crisbeto]

This was addressed in #15077. Closing.

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