fastboot: create Transport object (take 2).

(Second upload of this CL; original upload had the wrong version of
usb_windows.cpp that caused a compilation error. Fixed error and
re-tested.)

This CL creates a Transport object to provide a generic interface for
various transports. Specifically this is designed to be able to add UDP
support to fastboot in an upcoming CL without changing the main program
logic.

Also includes some minor code style fixes and replaces malloc/free
in the USB implementation files with smart pointers and std::string.

Bug: http://b/22029765
Change-Id: I1175bbce08690fbd15f51e68166be9b3e9973ea0