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.

Keep coverage.em file name for host rules where Jack is not called.

Bug: 27822188

(cherry picked from commit 491c9ce4
&& commit 8f0e52c3)

Change-Id: I26beee5a8d42a2ef1d476661e4b591fb09f1276f