MethodHandles: simplify findVirtual.

The code looks cleaner if we just return null from
Class.getInstanceMethod if a given method is not found, and then
throw the NSME from MethodHandles.findVirtual.

Addresses review comments on change cc44ded0.

Bug: 30550796
Test: make test-art-host
Change-Id: Id59258d3ff2a1a6a0f4b0cbc4fab4285b1703859