Add target.vendor.override_export_include_dirs.

This property was originally called export_include_dirs,
but because it has different semantics, it is renamed
here to override_export_include_dirs.

Bug: 62878521
Test: soong finds targets which use this property properly
Change-Id: Iac7c2a89ee8e389977c7a8e3b8c03c1a0d1608bb