Adding a timing for the scale animation in HelloWorld sample

[tbombach]

The animated value _scaleValue indicates that the size of the "Hello World" text should be animated, but there's no timing for it. Right now, the text just moves vertically, it doesn't scale as well.

This PR:

• Adds a timing for the _scaleValue animated value for Hello World text
• Combines the scale and transform timings in a composite parallel animation

Screen capture of the animation after this change: https://1drv.ms/v/s!Al9jOUS7_zLMiLZRxgwVGtqEFZkDgA

[msftclas]

@tbombach,
Thanks for your contribution as a Microsoft full-time employee or intern. You do not need to sign a CLA.
Thanks,
Microsoft Pull Request Bot