Verify java.lang.invoke.MethodHandle/Impl against OpenJDK 8u121-b13

Changes were non-functional and simply added/updated comments to
provide more details of the changes that were made.

The MethodHandleImpl is not based on the upstream code at all and so
has its own copyright message and a single change marker at the top of
the class explaining it.

Bug: 74379469
Test: make checkbuild, flash and start up device
Change-Id: I7ce80e178ab47d81c893fbb684bf5a0d82625896