Remove magic implementation selection based on test_for

As part of simplifiying the way dependencies are selected and removing
ApexInfoMutator, remove the automatic selection of implementation
over stubs when building tests against libraries in apexes.  Tests
can manually select linking against implementations via "libfoo#impl".

Bug: 372543712
Test: builds
Flag: EXEMPT refactor
Change-Id: I927db38a5691a1cf61f25fa02936c87cd36306f4