fbd-feedback-manager: fix device-specific theme path assembly

this picks up on !55 (closed) and uses g_build_filename to assemble a valid theme path from the bits given in fbd-feedback-manager. It introduces another temporary variable which is necessary to prevent g_build_filepath from mangling the actual filename by inserting a file path separator before the file ending (i.e. it would turn to some-file/.json otherwise, instead of some-file.json).

I'm new to open-source contributing and I really don't mean to be rude, but due to their last message I assumed the author of !55 (closed) didn't plan to follow up on your suggestions, so I made this one instead. I hope that's okay...

Curious to hear what you think. If you're satisfied with the addition to the README, I could add parts of it to the feedbackd-device-themes README, too.