Use getRelativeDayString in getRelativeTimeSpanString.

The bug we're fixing here is that languages that don't
distinguish the "one" case grammatically (such as Japanese)
would say the equivalent of "In 1 day" rather than "Tomorrow"
because of the misuse of getQuantityString.

This has the side-effect of switching us over to the CLDR
strings for relative day names, which have consistent capitalization;
the Android donottranslate-cldr.xml strings varied even within
a language, so although this is a change, it seems like a step
in the right direction.

In a future change, we should actually push all relative
day formatting down into icu4c.

Bug: 7098707
Change-Id: Ia2f9af3d18c441d6093dd5da7956a3d0130e5b06