Synchronous adb shell and pull for loopback start script

Before the removal and copy of script of video file on the android
device was done asynchronously, which was a bug.

BUG=webrtc:6545
NOTRY=True

Review-Url: https://codereview.webrtc.org/2470663004
Cr-Commit-Position: refs/heads/master@{#14898}

webrtc/examples/androidapp/start_loopback_stubbed_camera_saved_video_out.py

@@ -74,7 +74,7 @@ def main():
 
   # Delete output video file.
   if videoout:
-    subprocess.Popen(['adb', '-s', devname, 'shell', 'rm',
+    subprocess.check_call(['adb', '-s', devname, 'shell', 'rm',
         videoout])
 
   device = MonkeyRunner.waitForConnection(2, devname)
@@ -116,7 +116,7 @@ def main():
   if videooutsave:
     time.sleep(2)
 
-    subprocess.Popen(['adb', '-s', devname, 'pull',
+    subprocess.check_call(['adb', '-s', devname, 'pull',
         videoout, videooutsave])
 
 if __name__ == '__main__':