Agent libraries need to be searched for JNI functions

This makes agent libraries the option of last resort for native
method implementations. This will allow one to not need to manually
link all native methods in an agent library.

Bug: 37522517
Bug: 37432636
Test: ./test.py --host -j40
Change-Id: I5ad78a15e7e2799d2a877c5d603342899e2a1bd1