Trace.h: explicitly use global namespace

Imagine an AIDL file:
  package android.foo;
  interface IFoo {}

The AIDL C++ backend will generate classes like "::android::foo::IFoo".
In order to avoid ODR conflict, the NDK (Stable C) backend linking
against libbinder_ndk puts everything under the "::aidl" namespace. So,
we have "::aidl::android::foo::IFoo". When using ScopedTrace in a class
implementing a class from this backend, there is this problem.

Bug: 141828236
Test: works
Change-Id: Iafadea11788d4c011229657b4f53063bcf65f8d8