AAPT2: Record public status in a more robust way

This allows us to store the source and comments of a resource's
public declaration and avoids issues where there is no default
configuration for a publicly declared resource (like with drawables
of various densities) and AAPT2 mistakenly took this as an error.

Change-Id: I07a2fe9f551daefcce842f205fb219d2fa453ebc