Fixes for plural handling

* A new protected method looks for explicitly defined forms. Every
  overriden language class is required to use this method.
* Make explicit plural forms work for Russian. Russian has overridden
  convertPlural method, that was not taking care of explicit plural
  forms.
* Plural rules were updated to CLDR 24 for Manx (gv).

Bug: 46422
Bug: 47099
Bug: 54514
Change-Id: I46ab3dadc7fe08c1e60bbd81a1ee841e166e9608
Merge "Fixes for plural handling" into REL1_22