Clean up Handle usage.

Prefer using ConstHandle instead of Handle as function arguments
since you can't assign new references to ConstHandle which helps
prevent bugs.

Changed NullHandle to be a ConstHandle so that you can never modify
it to be a non null reference.

Change-Id: I81cb979f6f8d5b49e5614966a2caf28c3701dd4f
16 files changed