fix(stepper): Fix visual flake in focusing stepper header

[mitchellwills]

When the step is changed through a button click sometimes the focus of the new
step header is attributed by focus monitor to 'mouse' instead of 'program'. This
results in a visual difference in the step header (focus background is missing).
Instead, when focusing the step header focus it via 'program' explicitly through
the focus monitor.

[jelbourn]

@mmalerba could you take a look at this PR?

[ngbot]

I see that you just added the pr: merge ready label, but the following checks are still failing:
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[mitchellwills]

I updated the public API file to take into account the "api change" (not sure if this counts as a change since it's already inherited as a public method from the parent).
Let me know if this wasn't the right fix for the test

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