kiji-express-music: EXP-311. Rename Cell to FlowCell.

issue: https://jira.kiji.org/browse/EXP-311
review: https://review.kiji.org/r/1398
commit: https://github.com/kijiproject/kiji-express-music/commit/b640bc1c66b033ce9a261578506a70e8fafe688e