Adding a locale picker with two-step selection: language, then region.

This is intended to replace the UI part of the LocalePicker, at
least for the main "Settings" application (under "Language & Input")
and for the "Setup Wizard".

The UI part in the old LocalePicker can be removed once all the
components that use it are migrated to the new one.

Bug: 25800339
Change-Id: I2f264cdb7b7fc586124dd58fad2c68060d5b7aa8