Generated headers may not be arch specific

Previously, it was assumed that generated headers must be arch specific
and so prevented the fields referencing the paths to those headers from
being automatically optimized by the sdk generation code. That is not
always the case, e.g. with headers generated from protos so this change
allows those fields to be optimized.

Bug: 180427921
Test: m nothing
Change-Id: Id2af419d58ae3c30ea6d9e87f71e33a9ff6ba13b