Actually use IMA proguard configuration (oops!)

Issue: #3723

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=191407560

extensions/ima/build.gradle

@@ -21,7 +21,7 @@ android {
     defaultConfig {
         minSdkVersion project.ext.minSdkVersion
         targetSdkVersion project.ext.targetSdkVersion
-        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
+        consumerProguardFiles 'proguard-rules.txt'
     }
 }