Ensure consistent handling of generated headers/dirs

The list of exported generated headers are all expected to be within
one of the exported generated include dirs. Previously, that was not
the case as ExportedGeneratedIncludeDirs was suitable for extracting
to common properties (which changes the output location) and
exportedGeneratedHeaders was not.

This would cause a problem if there was only one variant. In that case
the ExportedGeneratedIncludeDirs would be treated as a common property
and placed in include_gen/<x> directory while exportedGeneratedHeaders
would be treated as an arch specific property and placed in
<arch>/include_gen/<x>.

Bug: 142935992
Test: m nothing
Change-Id: Idf82a5ca551b44ec31971c7ff3bd957a4c38f396