Floss: Add DBus logging for inbound traffic

Inbound and outbound DBus traffic should be logged if it's not noisy,
as it contains critical debugging information.

This CL enables logging the inbound traffic.

Bug: 298149797
Tag: #floss
Test: Manually observe bluetooth.log
Test: mma -j

Change-Id: I370dbbc1bb369feb3947c2fb916114567063eacc