Move nio direct byte buffers onto the Java heap.

Move nio direct byte buffers onto the Java heap.

Specifically, implement VMRuntime.newNonMovableArray and VMRuntime.addressOf.
For now these are pretty trivial because we don't have a copying collector,
but this (a) prevents code from allocating arbitrary amounts of direct byte
buffer without being charged for it, and (b) lets us move to a copying
collector in future.

Change-Id: I5de156dc8ac4ab5c997e18d447f635eb01ff08ab