Effect: Automatically save bitmaps in pixel test.

Also, omit the "actual" label from output files, as this boilerplate isn't necessary
(it doesn't disambiguate between any other saved filename like "expected").

PiperOrigin-RevId: 502378188

testdata/src/test/assets/media/bitmap/README.md

@@ -23,7 +23,7 @@ To generate new "expected" assets:
 
     ```shell
     adb pull \
-    /sdcard/Android/data/com.google.android.exoplayer2.effect.test/cache/drawFrame_rotate90_actual.png \
+    /sdcard/Android/data/com.google.android.exoplayer2.effect.test/cache/drawFrame_rotate90.png \
     third_party/java_src/android_libs/media/libraries/test_data/src/test/assets/media/bitmap/sample_mp4_first_frame/electrical_colors/rotate90.png
     ```
 

testutils/src/main/java/com/google/android/exoplayer2/testutil/BitmapPixelTestUtil.java

@@ -177,9 +177,8 @@ public class BitmapPixelTestUtil {
    */
   public static void maybeSaveTestBitmapToCacheDirectory(
       String testId, String bitmapLabel, Bitmap bitmap) {
-    File file =
-        new File(
-            getApplicationContext().getExternalCacheDir(), testId + "_" + bitmapLabel + ".png");
+    String fileName = testId + (bitmapLabel.isEmpty() ? "" : "_" + bitmapLabel) + ".png";
+    File file = new File(getApplicationContext().getExternalCacheDir(), fileName);
     try (FileOutputStream outputStream = new FileOutputStream(file)) {
       bitmap.compress(Bitmap.CompressFormat.PNG, /* quality= */ 100, outputStream);
     } catch (IOException e) {