Fix generated headers of snapshot

For VNDK snapshot and SDK snapshot, deps files have been used to capture
generated headers. But exported deps might contain intermediate phony
files instead of actual header files, which are for optimization of
ninja. To correctly capture all headers, exported generated header files
are gathered separately.

Bug: 65377115
Test: m nothing
Change-Id: Ia03fa69186490a818578190e3c0bfb0261d1fd6e