Fix for coverage collection. (#490)

Make coverage task depend on only firebase product check tasks.

Author: yifanyang

tools/measurement/coverage/coverage.gradle

@@ -13,6 +13,7 @@
 // limitations under the License.
 
 
+import com.google.firebase.gradle.plugins.FirebaseLibraryExtension
 import com.google.firebase.gradle.plugins.measurement.coverage.GenerateMeasurementsTask
 import com.google.firebase.gradle.plugins.measurement.UploadMeasurementsTask
 
@@ -21,7 +22,11 @@ task generateCoverageMeasurements(type: GenerateMeasurementsTask) {
     description 'Runs checkCoverage task in all projects and calculates coverage percents.'
     group 'Measurements'
 
-    dependsOn rootProject.allprojects.collect { it.tasks.withType(JacocoReport) }.flatten()
+    dependsOn rootProject.allprojects.findAll {
+        it.extensions.findByType(FirebaseLibraryExtension) != null
+    }.collect {
+        it.tasks.withType(JacocoReport)
+    }.flatten()
     reportFile = file("$buildDir/coverage-report.json")
 }