Call command handler with weak_ptr

Command handler should keep only weak_ptr to command and check on every
access. Long async commands will need to keep reference to the command.

BUG:24267885

Change-Id: I3ca3be9e31b9e9a942eca001ed21f1133973f0ea
Reviewed-on: https://weave-review.googlesource.com/1255
Reviewed-by: Alex Vakulenko <avakulenko@google.com>