Android: Guard against switching camera on stopped camera

It is possible that cameraThreadHandler is null upon calling
switchCamera(). This CL adds a guard against that.

Review URL: https://codereview.webrtc.org/1325333003

Cr-Commit-Position: refs/heads/master@{#9925}
diff --git a/AUTHORS b/AUTHORS
index 5ffa000..2398b88 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -40,5 +40,6 @@
 Mozilla Foundation
 Opera Software ASA
 struktur AG
+Telenor Digital AS
 Temasys Communications
 Vonage Holdings Corp.