floss: dbus_proxy_obj: Apply DBusArg::log to the args

This was missed so for outging DBus logs we are actually using the
default formatter. This is harmless but would be a blocker of
https://r.android.com/3102963.

This patch also fixed some diagnosis warnings.

Bug: 342337056
Tag: #floss
Test: mmm packages/modules/Bluetooth
Flag: EXEMPT, Floss-only changes
Change-Id: I355e215d6a74c0e299088b32d053864ab97d9061