Model token formats supported in newer Android API levels

[JakeWharton]

• In API 29, Android uses ICU version 63.2 whose MessageFormat supports the skeleton text format of its new NumberFormatter class for the number type.
• In API 30, Android uses ICU version 66.1 whose MessageFormat supports skeleton text format via DateFormat.getInstanceForSkeleton for the date type.

If you use these types, we should emit @RequiresApi(29/30) on the resulting functions.

[drewhamilton]

DateFormat.getInstanceForSkeleton has been around since API 24. Does MessageFormat just not use it in APIs 24-29?

[drewhamilton]

Can confirm a literal :: is printed on APIs 24-29.

[JakeWharton]

Yeah I was just going on the docs. I did not test anything.

[drewhamilton]

We'll need to check whether the resource containing :: has OS level variants. E.g. if somebody has:

values/strings.xml:
  <string name="foo">{date, date, yyyy-MM-dd}</string>

values-v30/strings.xml:
  <string name="foo">{date, date, ::yyyy-MM-dd}</string>

then they shouldn't get a @RequiresApi annotation.