Fix Jacoco coverage runner.

- Correctly set the version of Jacoco to use via classpath dependency in
  build.gradle as noted in JacocoOptions.java
- Move to using jacoco 0.7.8

Bug: 37475483
Test: ./gradlew assembleAndroidTest and locally verify that dist/ and test apks contain
      correct version of Jacoco.

Change-Id: I2577bdaf3d5ba31102191ac0048595e042513618
diff --git a/build.gradle b/build.gradle
index 64a08a2..5bd5d13 100644
--- a/build.gradle
+++ b/build.gradle
@@ -25,6 +25,7 @@
 
     dependencies {
         classpath libs.gradle
+        classpath libs.jacoco
     }
 }