Improve library lookup logic

Linker tries to open a library even if it can
be found by soname. This only happens if the
library was previously opened under different
target sdk version.

Bug: http://b/21876587
Bug: http://b/21153477
Bug: http://b/21171302
Bug: https://code.google.com/p/android/issues/detail?id=160921
Change-Id: I769a04b6b1368a107d43f399297be14050338bbc