libbase: fix libbase_test.logging.StdioLogger.

I don't think the StdioLogger test ever passed. Move GetFileBasename
to where we can use it from StdioLogger, and undo the mix of anonymous
namespace vs static to consistently use static (which is the majority)
while we're touching those lines.

Bug: N/A
Test: ran tests
Change-Id: I95b3966cdb8af642bed71752bd7d4e3a86ac84ca