LayoutLib: Correctly resolve ?attr/foo [DO NOT MERGE]

?attr/foo in platform files wasn't resolved properly. The IDE checked
the app namespace for the attribute even though it is being referenced
from a platform file.

Change-Id: I7db6dfef5b540d99804c298120578da9d060b886
(cherry picked from commit e21c7ec8a02922a6bec19436897f9ee02be3d6c2)