Add a way to force a frame to be sent from SurfaceTextureHelper

This allows forcing a minimum frame rate if the producer doesn’t update
the SurfaceTexture often.

This needs to be done in SurfaceTextureHelper to keep the
synchronization of the texture access consistent.

Bug: b/149383039
Change-Id: I0e3c82dd51d486b931bd8dda0fd9d5cdb1a90901
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/177001
Reviewed-by: Sami Kalliomäki <sakal@webrtc.org>
Commit-Queue: Xavier Lepaul <xalep@google.com>
Cr-Commit-Position: refs/heads/master@{#31504}

sdk/android/api/org/webrtc/SurfaceTextureHelper.java

@@ -263,6 +263,17 @@ public class SurfaceTextureHelper {
     });
   }
 
+  /**
+   * Forces a frame to be produced. If no new frame is available, the last frame is sent to the
+   * listener again.
+   */
+  public void forceFrame() {
+    handler.post(() -> {
+      hasPendingTexture = true;
+      tryDeliverTextureFrame();
+    });
+  }
+
   /** Set the rotation of the delivered frames. */
   public void setFrameRotation(int rotation) {
     handler.post(() -> this.frameRotation = rotation);