Replace Locale with LocaleList in LocaleSpan.

This is a straightforward upgrading of LocaleSpan.

LocaleSpan was designed to be a mechanism to override text locale only
in a certain substring.  Since now text locale supports multiple locales
with LocaleList, it should make sense to let LocaleSpan have the ability
to have multiple locales with LocaleList.

This CL also addresses an existing issue that LocaleSpan maintained only
Language/Country/Variant tags.  Now Locale data associated with a
LocaleSpan is guaranteed to be preserved as long as each of them can be
created from a BCP 47 language tag with Locale#forLanguageTag(String).

Bug: 26567158
Change-Id: Ib754796d399cd00f9d02005df88874a5cd5c7642