Fix BufferTest#testFreed.

Each DirectByteBuffer now has a MemoryRef object associated with it.
This MemoryRef is shared across its duplicates and slices.

Also contains a few other surrounding cleanups :
- FileChannelImpl now allocates the DirectByteBuffer directly.
- Removed a pointless constructor from DirectByteBuffer.
- Removed unnecessary (pos << 0) shifts.

bug: 26020981
Change-Id: I573b48e621667c3cc2d78287678fba240783603d