[datetime] fix(TimePicker): improve a11y markup

[bvandercar-vt]

Fixes #0000

Checklist

• [N/A] Includes tests
• [N/A] Update documentation

Changes proposed in this pull request:

Reviewers should focus on:

Spinbutton example: https://w3c.github.io/aria-practices/examples/spinbutton/datepicker-spinbuttons.html

[adidahiya]

it's good that you included the time unit here, but it looks like it will not print nicely for some of the units. can you create a helper function in timeUnit.ts to pretty print the time unit enum? something like:

/** Prints a descriptive label representing the plural of th given time unit. */
export function printTimeUnit(unit: TimeUnit) {
    return {
        [TimeUnit.HOUR_24]: "hours (24hr clock)",
        [TimeUnit.HOUR_12]: "hours (12hr clock)",
        [TimeUnit.MINUTE]: "minutes",
        [TimeUnit.SECOND]: "seconds",
        [TimeUnit.MS]: "milliseconds",
    }[unit];
}

... and then use that helper function in this label value.

[bvandercar-vt]

done