java.lang.invoke: Add MethodHandle.

- Addresses TODOs in MethodType that depended on the inclusion
  of java.lang.invoke.MethodHandle.

- None of the functionality in this change is functional or testable
  currently. We need this because there are a few circular dependencies
  between the runtime implementation of polymorphic invokes and this
  class.

- This change also adds dummy native implementations of invoke /
  invokeExact. These methods must be marked as native and must throw
  an UnsupportedOperation when invoked reflectively.

bug: 30550796
test: make docs

Change-Id: Ic9c3faa4be4fd3b443b9dab8f7c4f5e8f8c1187a
6 files changed