Enable test coverage report generation for library project.

To generate the reports, under 'v2' folder run:
./gradlew createDebugCoverageReport

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=129991223

library/build.gradle

@@ -35,6 +35,9 @@ android {
             minifyEnabled false
             proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
         }
+        debug {
+            testCoverageEnabled = true
+        }
     }
 
     lintOptions {