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.

Bug: 130138217
Test: manual build + TreeHugger
Merged-In: Ic7931f990369f744c8de62956a1a0a9c0451d6ab
Change-Id: Ic7931f990369f744c8de62956a1a0a9c0451d6ab
(cherry picked from commit dca3186b)