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
diff --git a/core/java.mk b/core/java.mk
index 811f7ca..74e5875 100644
--- a/core/java.mk
+++ b/core/java.mk
@@ -448,7 +448,7 @@
 endif
 
 ifeq ($(LOCAL_EMMA_INSTRUMENT),true)
-$(full_classes_emma_jar): PRIVATE_EMMA_COVERAGE_FILE := $(intermediates.COMMON)/coverage.em
+$(full_classes_emma_jar): PRIVATE_EMMA_COVERAGE_FILE := $(intermediates.COMMON)/coverage.emma.ignore
 $(full_classes_emma_jar): PRIVATE_EMMA_INTERMEDIATES_DIR := $(emma_intermediates_dir)
 # module level coverage filter can be defined using LOCAL_EMMA_COVERAGE_FILTER
 # in Android.mk