Declare main outside of extern "C" block

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])."

Bug: http://b/379133546
Test: treehugger
Change-Id: I0a6809b9671089fad24094e903edbb7a6704fbca