Create withMediaPipe variant only if AAR is present

This should fix running `./gradlew clean test` if MediaPipe hasn't been built, for example.

PiperOrigin-RevId: 452033698

demos/transformer/README.md

@@ -57,8 +57,9 @@ manual steps.
       ${TRANSFORMER_DEMO_ROOT}/src/withMediaPipe/assets
     ```
 
-1.  Select the `withMediaPipe` build variant in Android Studio, then build and
-    run the demo app and select a MediaPipe-based effect.
+1.  In Android Studio, gradle sync and select the `withMediaPipe` build variant
+    (this will only appear if the AAR is present), then build and run the demo
+    app and select a MediaPipe-based effect.
 
 [Transformer]: https://exoplayer.dev/transforming-media.html
 [MediaPipe]: https://google.github.io/mediapipe/

demos/transformer/build.gradle

@@ -56,6 +56,16 @@ android {
             dimension "mediaPipe"
         }
     }
+
+    // Ignore the withMediaPipe variant if the MediaPipe AAR is not present.
+    if (!project.file("libs/edge_detector_mediapipe_aar.aar").exists()) {
+        variantFilter { variant ->
+            def names = variant.flavors*.name
+            if (names.contains("withMediaPipe")) {
+                setIgnore(true)
+            }
+        }
+    }
 }
 
 dependencies {