Fix coverage build after adding clang coverage.

Static libraries are painful. Details are in the comment. I'll try to
get prebuilts in to the tree so I can fix this on Monday. Until then,
this isn't actually a regression because we've not had coverage
available for this library until now anyway.

Bug: 17574078
Change-Id: I7505c8a94007203e15a6cf192caa06004849d7d9