Treat file paths as UTF-8

Chrome has a class to make it easier to work with file paths in a cross
platform fashion.

Unfortunately this class exposes different base types under windows (std::wstring vs
std::string).

We now assume that all file paths are utf-8 encoded (true for posix), so
we can use std::string for both Win32 and posix.

Bug: 186567864
Change-Id: Idb1389ce19502599a7fd4fd60a62c99b9a318e93
Test: Launch rootcanal, see gDevice beacon show up