GL: Reduce visibility of GlProgram.loadAsset.

This is only used within the same class, so can be private.

PiperOrigin-RevId: 514665000

library/common/src/main/java/com/google/android/exoplayer2/util/GlProgram.java

@@ -66,7 +66,7 @@ public final class GlProgram {
    * @return The content of the file to load.
    * @throws IOException If the file couldn't be read.
    */
-  public static String loadAsset(Context context, String assetPath) throws IOException {
+  private static String loadAsset(Context context, String assetPath) throws IOException {
     @Nullable InputStream inputStream = null;
     try {
       inputStream = context.getAssets().open(assetPath);