Code tidy ups for the ByteBuffer implementations

HeapByteBuffer can be marked final.
DirectByteBuffer cannot be final, but we can declare most
methods final to make it easier to reason about.

Also:
Marked overridden methods with @Override.
Added @SuppressWarnings("unused") on a method invoked via
JNI.
Removed unused methods / constructors.
Removed unnecessary casts.
Added comments where needed.
Minor whitespace changes.

No functional changes intended.

Test: TreeHugger compile
Change-Id: I40751d5066bbafc42c8af1495bd7576fe14d419f