AAPT2: Support static lib referencing static lib

When a static library A references static library B,
and app C references both A and B, we get the following symbol merging,
symbols from library B get imported twice.

We must only check that symbol references to library B are valid
when building library A. We should only merge all the symbols
when building final app C.

Change-Id: I23cba33b0901dcbb5328d9c9dfaa6a979c073c36