[LinearProgress] Reduce delay before indeterminate animation begins

[lawrence-yu]

With how the indeterminate linear progress is currently implemented, when the animation is kicked off at componentDidMount, there's a one-time extra 420ms delay added on before we see the first bar go by (which, to me, makes it look like an abnormally long time before it starts - compared with the rest of the animation cycle once it gets going).

This change shaves off this delay while not altering the timing of the rest of the animation.

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
• Description explains the issue / use-case resolved, and auto-closes the related issue(s) (http://tr.im/vFqem).

[oliviertassinari]

I couldn't noticed any visual difference, but I think you are right with the approach.

[oliviertassinari]

@lawrence-yu Thanks.