Move locale negotiation to ResourcesManager

Previously, locale resolution happened at Resources level, which
could result in different Resources in the same app using different
locales.

This change moves the locale negotiation to ResourcesManager, which
remembers the locales supported by each Resources as they are
created, and negotiates the locale based on their union.

It also makes sure that the old model of apps getting the default
locale by accessing it through the Configuration's locale attribute
continues to work, since the negotiated locale will be moved to the
top of the list before Resources are created and when they are
updated with a Configuration change.

Bug: 25800576
Bug: 26404653
Change-Id: I0dd81df288327fd8d1229188c9544ee2209c161a