Add android_debug_unopt_arm64 to local_engine.json. (#51427)

As written:

```sh
flutter % et query builders | grep unopt_arm64                           
   "android_debug_unopt_arm64" config
   "host_debug_unopt_arm64" config
```

Yay!

Author: matanlurey

ci/builders/local_engine.json

@@ -19,6 +19,26 @@
         "targets": []
       }
     },
+    {
+      "drone_dimensions": [
+        "os=Mac-13",
+        "os=Linux"
+      ],
+      "gn": [
+        "--runtime-mode",
+        "debug",
+        "--unoptimized",
+        "--android",
+        "--android-cpu=arm64",
+        "--no-stripped",
+        "--no-lto"
+      ],
+      "name": "android_debug_unopt_arm64",
+      "ninja": {
+        "config": "android_debug_unopt_arm64",
+        "targets": []
+      }
+    },
     {
       "drone_dimensions": [
         "os=Mac-13",