Rename emma coverage files

We no longer use emma with Jack. However, during the process of a
full build, we may run the emma task and overwrite the coverage file
generated by Jack with the one from emma because they use the same
filename.

This CL renames coverage files generated by emma so they no longer
break those created by Jack.

Bug: 27822188
Change-Id: Ie3fdc2b8f53deafbefbd6973e39f29d8c46f371e