binder: declare main in global NS, w/o extern "C"

The main function should be in the global namespace, not the android
namespace. We were using 'extern "C"' to prevent the mangling, but the
C++ standard doesn't allow a linkage-specification for main,
https://eel.is/c++draft/basic.start.main#3.sentence-5:

"The main function shall not be declared with a linkage-specification
([dcl.link])."

Clang has started warning on incorrect declarations of main (-Wmain).

Bug: http://b/379133546
Test: m binder_thread_stats libbinderdebug_test binderSafeInterfaceTest
Change-Id: I1c53ce621391b5577c8c04b21117e9b59fe05136