Rename startTransformation to start

Also update some Javadoc in Transformer

PiperOrigin-RevId: 507395956

demos/transformer/src/main/java/androidx/media3/demo/transformer/TransformerActivity.java

@@ -213,7 +213,7 @@ public final class TransformerActivity extends AppCompatActivity {
       Transformer transformer = createTransformer(bundle, filePath);
       EditedMediaItem editedMediaItem = createEditedMediaItem(mediaItem, bundle);
       transformationStopwatch.start();
-      transformer.startTransformation(editedMediaItem, filePath);
+      transformer.start(editedMediaItem, filePath);
       this.transformer = transformer;
     } catch (PackageManager.NameNotFoundException e) {
       throw new IllegalStateException(e);

libraries/transformer/src/androidTest/java/androidx/media3/transformer/TransformerAndroidTestRunner.java

@@ -231,8 +231,7 @@ public class TransformerAndroidTestRunner {
    * @param testId An identifier for the test.
    * @param editedMediaItem The {@link EditedMediaItem} to transform.
    * @return The {@link TransformationTestResult}.
-   * @throws IllegalStateException See {@link Transformer#startTransformation(EditedMediaItem,
-   *     String)}.
+   * @throws IllegalStateException See {@link Transformer#start(EditedMediaItem, String)}.
    * @throws InterruptedException If the thread is interrupted whilst waiting for transformer to
    *     complete.
    * @throws IOException If an error occurs opening the output file for writing.
@@ -313,8 +312,7 @@ public class TransformerAndroidTestRunner {
         .runOnMainSync(
             () -> {
               try {
-                testTransformer.startTransformation(
-                    editedMediaItem, outputVideoFile.getAbsolutePath());
+                testTransformer.start(editedMediaItem, outputVideoFile.getAbsolutePath());
                 // Catch all exceptions to report. Exceptions thrown here and not caught will NOT
                 // propagate.
               } catch (Exception e) {