[inspector_protocol] Fix building with non-ASCII paths

The Python code internally treats path as unicode strings, but the
command line options are read as byte strings. Mixing these up
does not work properly, so decode the CLI arguments after parsing.

(This was noticed because it makes the Node.js build fail under
paths containing non-ASCII characters.)

Bug:
Change-Id: Ibbedda314aec98969db9859fc92b152f2c9239af