Add ofSkeleton demo to demo app (#313)

* Clean up desugaring mentions in `AndroidDateTimeFormatter.ofSkeleton` doc comments
* Pull standard format demo into standalone function
* Add tab for skeleton format demo
* Update Demo API to take `ZonedDateTime` rather than `Instant`
* Move system time setting in Demo to take up less space
* Reorder functions in Demo.kt to match UI structure
* Remember all date-time formatters
* Make a bunch of demo properties saveable
* Fix state saving of locale dropdown
* Hide locale dropdown when there is only one suggestion matching the typed text

Author: drewhamilton

datetimeformatters/src/main/java/dev/drewhamilton/androidtime/format/AndroidDateTimeFormatter.kt

@@ -431,8 +431,7 @@ object AndroidDateTimeFormatter {
      *
      * This function will automatically correct for grammatical necessity. Given the same "MMMMd"
      * input, the formatter will use "d LLLL" in the `fa_IR` locale, where stand-alone months are
-     * necessary. **Warning: core library desugaring does not currently support formatting with
-     * 'L'.**
+     * necessary.
      *
      * Lengths are preserved where meaningful, so "Md" would give a different result to "MMMd", say,
      * except in a locale such as `ja_JP` where there is only one length of month.
@@ -463,8 +462,7 @@ object AndroidDateTimeFormatter {
      *
      * This function will automatically correct for grammatical necessity. Given the same "MMMMd"
      * input, the formatter will use "d LLLL" in the `fa_IR` locale, where stand-alone months are
-     * necessary. **Warning: core library desugaring does not currently support formatting with
-     * 'L'.**
+     * necessary.
      *
      * Lengths are preserved where meaningful, so "Md" would give a different result to "MMMd", say,
      * except in a locale such as `ja_JP` where there is only one length of month.