Handle explicit plural forms in custom convertPlural in language classes

A new protected method looks for explicitly defined forms.
Every overriden language class is required to use this method.

Includes tests.

Redoing old patch I6dc759e3dfb05d6673209ba00da6592a384d5300

Bug: 46422
Change-Id: I2a9f93567087babb896999f1214d3c56afc67c96
Merge "Handle explicit plural forms in custom convertPlural in language classes"