Add ro.product.locale

This is a BCP-47 language tag representing the default locale for a
given device. Support for the earlier mechanism of supplying a language
and region via ro.product.locale.region/language has been removed. This
change also removes support for supplying these properties via
PRODUCT_ADDITIONAL_PROPERTY_OVERRIDES. Devices that need to override
this should either rearrange their PRODUCT_LANGUAGES or supply a custom
.prop file.

bug: 17691569

Change-Id: I00c74098542b49b9c514a6ca39ea8d08179546c1