Replace the android support annotation library with androidx's one.

This change does not affect downstream dependencies as androidx.annotation
is fully compatible with android.support.annotation.

Bug: webrtc:11962
Change-Id: I714b473df8d0fee8000ddf3a9beca7c5613db5ff
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/226881
Commit-Queue: Xavier Lepaul‎ <xalep@webrtc.org>
Reviewed-by: Xavier Lepaul‎ <xalep@webrtc.org>
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#34839}

examples/unityplugin/java/src/org/webrtc/UnityUtility.java

@@ -11,7 +11,7 @@
 package org.webrtc;
 
 import android.content.Context;
-import android.support.annotation.Nullable;
+import androidx.annotation.Nullable;
 
 public class UnityUtility {
   private static final String VIDEO_CAPTURER_THREAD_NAME = "VideoCapturerThread";