Rename default select branch to 'default' keyword

Previously I was using an underscore to denote the default branch
because I was thinking that I would allow variable bindings in the
select branches, and 'default' could be mistaken for the name of a
variable. But I think it's better to just introduce alternate syntax,
like `default @ my_var: "foo" + my_var,` to do the variable bindings,
so that we can have a clearer name for the default case.

Bug: 323382414
Test: m nothing --no-skip-soong-tests
Change-Id: I8cbded97a029c06782fb5313d8c0e67f22951482