stub variants also re-exports headers

This change fixes a bug that headers are not re-exported from stub
variants of a library, if the headers are not from header libs, but from
shared or static libs. This is because only header lib dependencies
are respected for stubs variants.

The fix is as follows. 1) dependencies to the shared/static libs are
added even for stubs variants. 2) instead, in depsToPaths, they are
treated like header libs (i.e. don't contribute to linkFile) for the
stubs variants.

Bug: 174558745
Test: m

Change-Id: Iab6c77e7817055d0f2d09cb114186b30164fc231