Enable jdiff for api-stubs-docs

And also clean legacy MK code in Android.mk

Test: api-stubs-docs-diff
Bug: b/78245848
Change-Id: I733e0e2c29f8273db3ffb9730c70d5b4e2b1438d
diff --git a/Android.bp b/Android.bp
index 3a5fcfa..da46917 100644
--- a/Android.bp
+++ b/Android.bp
@@ -1571,6 +1571,7 @@
             removed_api_file: "api/removed.txt",
         },
     },
+    jdiff_enabled: true,
 }
 
 droidstubs {
@@ -1595,6 +1596,7 @@
             removed_api_file: "api/system-removed.txt",
         },
     },
+    jdiff_enabled: true,
 }
 
 droidstubs {