Readd jacoco plugin and make coverage work (#728)

diff --git a/build.gradle b/build.gradle
index 4d4b749..acb3e08 100644
--- a/build.gradle
+++ b/build.gradle
@@ -44,6 +44,7 @@
     apply plugin: "maven"
     apply plugin: "signing"
     apply plugin: "idea"
+    apply plugin: "jacoco"
     apply plugin: "net.ltgt.errorprone"
 
     group = "org.conscrypt"
@@ -154,6 +155,10 @@
         }
     }
 
+    jacoco {
+        toolVersion = "0.8.4"
+    }
+
     task generateProperties(type: WriteProperties) {
         ext {
             parsedVersion = VersionNumber.parse(version)