[fidl] Fix the last usages of raw channels

A previous change migrated most use of LLCPP to typed channels but
missed a few. They were caught when we started conditionally deprecating
parts of the raw channel API in

This change fixes them until the code builds successfully. Unfortunately
the code gets slightly more verbose as a result, because many operations
interface with raw zx_handle_t during service discovery. Moving the
GetConnectToServiceFunction to look more similar to
`service::Connect<Protocol>` could simplify some of those in the future.

TEST: Fuchsia GI.
Change-Id: I3edbc488f0458a4cc461f588e708f1edc2d750b7
5 files changed