Enables ADBD tracing in the emulator.

When running inside the emulator, guest's adbd can connect to 'adb-debug' qemud
service that can display adb trace messages on condition that emulator has been
started with '-debug adb' option.

This CL enables that functionality in ADB code.

Change-Id: I59b4a76d3c887ad28b8aa8e2a01dfa814e75faa1