keychords.cpp: O_RDWR -> O_RDONLY

The keycodes functionality in init only needs to read key events. It
doesn't need to inject key events. A read-only file descriptor is
perfectly sufficient.

Test: pressing volume-up/volume-down/power still generates a bugreport
Change-Id: Iecbf21c544914334006ac6258c03df390f417e80