MethodHandles: Fix buggy findVirtual implementation.

MethodHandles.findVirtual doesn't map cleanly to any reflection API
so we'll have to reimplement in terms of a new helper in
java.lang.Class.

This change also fixes an issue where we wouldn't correctly deal with
virtual functions on java.lang.invoke.MethodHandle itself.

Test: make test-art-host + vogar ClassTest MethodHandlesTest
Bug: 30550796
Change-Id: I1948d48520a5960807696f926db80969aed6f58b