ART: Intrinsify polymorphic signature methods

Adds VarHandle accessor method to list of intrinsics.

Adds code to interpreter to ensure intrinsics with polymorphic
signatures are initialized.

Rename most uses of InvokePolymorphic to InvokeMethodHandle (and
similar changes) to be clear that the particular code path applies to
MethodHandle instances rather than VarHandle.

Change-Id: Ib74865124a1e986badc0a7c4bb3d782af07225d4
Bug: 65872996
Test: art/test.py --host