Emit diffs into offline SDK docs.

We already generate offline SDK docs as part of "docs" build, and
this change simply adds the "-diff" targets so they're also included.

Test: make -j48 docs dist
Bug: 69115817
Change-Id: Ic8b29b32c46feabf1951125edebe60ab8069e844
diff --git a/Android.mk b/Android.mk
index 7ca8358..10d11f3 100644
--- a/Android.mk
+++ b/Android.mk
@@ -656,6 +656,10 @@
 # Check comment when you are updating the API
 update-api: doc-comment-check-docs
 
+# Generate API diffs as part of docs builds
+docs: offline-sdk-referenceonly-diff
+docs: offline-system-sdk-referenceonly-diff
+
 # ====  static html in the sdk ==================================
 include $(CLEAR_VARS)