AsyncChannel: Migrate Slog to Log to enable Jarjaring

AsyncChannel & AsyncService both use Slog, which is
@hide, so jarjar'ing them would still lead to @hide API
usages.
Migrate to Log, which is public, so that AsyncChannel &
AsyncService can be jarjar'ed.

Bug: 145409537
Test: compiles
Change-Id: Ic0b9e8d66d575925cb1972e20721dd53c85307e6