Add Gradle task for generating API diffs

Also cleans up usage of deprecated leftShift operator.

Bug: 33487656
Test: ./gradlew generateDiffs
Change-Id: I680e3dfc3869d4b2235b23785a99368c9be69486
diff --git a/settings.gradle b/settings.gradle
index 16b7b91..dd64402 100644
--- a/settings.gradle
+++ b/settings.gradle
@@ -130,3 +130,6 @@
 
 include ':doclava'
 project(':doclava').projectDir = new File(externalRoot, 'doclava')
+
+include ':jdiff'
+project(':jdiff').projectDir = new File(externalRoot, 'jdiff')