Add external links for Camera API documentation (#3986)

Author: saudsami

en-US/dita/RTC-NG/config/keys-rtc-ng-links-android.ditamap

@@ -40,6 +40,20 @@
         </keydef>
     </topichead>
     <topichead navtitle="External">
+        <keydef keys="camera1-link" 
+            href="https://developer.android.google.cn/reference/android/hardware/Camera#open(int)" 
+            scope="external">       
+            <topicmeta>
+                <linktext>Camera.open()</linktext>
+            </topicmeta>
+        </keydef>
+        <keydef keys="camera2-link" 
+            href="https://developer.android.google.cn/reference/android/hardware/camera2/CameraManager?hl=en#getCameraIdList" 
+            scope="external">
+            <topicmeta>
+                <linktext>CameraManager.getCameraIdList()</linktext>
+            </topicmeta>
+        </keydef>
         <keydef keys="filePath-link"
             href="https://developer.android.com/guide/topics/media/media-formats?hl=zh-cn"
             scope="external" format="html"/>