Skip unneeded notice files for ETC modules.

Non-installable, non-library modules can still have notice files
attached if they are bundled in an apex module, in which case the
current make setting would generate an error. This change makes it just
ignore them silently if the module is ETC. Other classes will still
trigger an error.

Test: manual build + TreeHugger
Change-Id: Ic7931f990369f744c8de62956a1a0a9c0451d6ab