Change the license detection logic to return if a dual-license was
found. Update the metadata comment as discussed in b/179902335.
Add unit tests for add_module_license and decide_license_type.
Bug: 179902335
Test: atest --host add3prf_test; manually run add3prf.py in {ahash,
libloading, parking_lot, ring}
Change-Id: I3184b5cf0955d124170b9e5987308507bdef6e9a
