Add a stub for android.util.StatsEvent.

Needed for compile checks of platform tests in gradle, e.g.
for the Continuous Integration.  The actual test will only run
against the Android platform so a throwing stub is sufficient.

Also updated the errorprone plugin as the older one is
crashing for me.
diff --git a/build.gradle b/build.gradle
index 6682159..089531c 100644
--- a/build.gradle
+++ b/build.gradle
@@ -2,7 +2,7 @@
 
 buildscript {
     ext.android_tools = 'com.android.tools.build:gradle:4.0.0'
-    ext.errorproneVersion = '2.3.3'
+    ext.errorproneVersion = '2.4.0'
     ext.errorproneJavacVersion = '9+181-r4173-1'
     repositories {
         google()
@@ -20,7 +20,7 @@
     // Add dependency for build script so we can access Git from our
     // build script.
     id 'org.ajoberstar.grgit' version '3.1.1'
-    id 'net.ltgt.errorprone' version '1.1.1'
+    id 'net.ltgt.errorprone' version '1.3.0'
 }
 
 subprojects {