Add GYP target for WebRTC Video demo for Android.

Add a build target for the Video demo app for Android that only
exists when OS=='android' during build.

Note that this doesn't solve webrtc:1029, it's more like a workaround
waiting for the complete solution, which is to great a proper GYP target
that doesn't involve an action and an external script.

BUG=1029
TEST=Built successfully with:
source build/android/envsetup.sh
gclient runhooks
ninja -C out/Debug
Also verified the target is not present when OS is not 'android'.

Review URL: https://webrtc-codereview.appspot.com/1286004

git-svn-id: http://webrtc.googlecode.com/svn/trunk@3769 4adac7df-926f-26a2-2b94-8c16560cd09d

webrtc/video_engine/video_engine.gyp

@@ -18,6 +18,13 @@
         'test/libvietest/libvietest.gypi',
         'test/auto_test/vie_auto_test.gypi',
       ],
+      'conditions': [
+        ['OS=="android"', {
+          'includes': [
+            'test/android/android_video_demo.gypi',
+          ],
+        }],
+      ],
     }],
   ],
 }