ImageReader: Make discardFreeBuffers public

Test: CTS
Bug: 64847201
Change-Id: I61008111552b5ca4641a509e5bcf5cf821582d5e

api/current.txt

@@ -22028,6 +22028,7 @@ package android.media {
     method public android.media.Image acquireLatestImage();
     method public android.media.Image acquireNextImage();
     method public void close();
+    method public void discardFreeBuffers();
     method public int getHeight();
     method public int getImageFormat();
     method public int getMaxImages();

media/java/android/media/ImageReader.java

@@ -640,7 +640,6 @@ public class ImageReader implements AutoCloseable {
      * The ImageReader continues to be usable after this call, but may need to reallocate buffers
      * when more buffers are needed for rendering.
      * </p>
-     * @hide
      */
     public void discardFreeBuffers() {
         synchronized (mCloseLock) {