RPC Binder: stronger type checks for AsyncTodo

I was confused why we had a comment saying to only use
local binders when we can use the type to do this. Not
sure what I was thinking, and this confused me when
working on more recent memory issues.

Bug: 416772676
Test: binderRpcTest
Change-Id: I63b15567d904ccfa28a907e8b927d4ce9a8e98e7