Do not crash when receiving video call on device w/o camera

Upon accepting a video call on a device that can not establish a video track on
its own (for example by not having a camera), displaying the video enable/disable
button would fail. This commit defaults this button to disabled.

(cherry picked from commit 1822a71c2a80ce31dac1b8a4df25acf012171b43)

src/main/java/eu/siacs/conversations/ui/RtpSessionActivity.java

@@ -888,7 +888,12 @@ public class RtpSessionActivity extends XmppActivity implements XmppConnectionSe
     }
 
     private void enableVideo(View view) {
-        requireRtpConnection().setVideoEnabled(true);
+        try {
+            requireRtpConnection().setVideoEnabled(true);
+        } catch (final IllegalStateException e) {
+            Toast.makeText(this, R.string.unable_to_enable_video, Toast.LENGTH_SHORT).show();
+            return;
+        }
         updateInCallButtonConfigurationVideo(true, requireRtpConnection().isCameraSwitchable());
     }
 

src/main/java/eu/siacs/conversations/xmpp/jingle/WebRTCWrapper.java

@@ -393,7 +393,7 @@ public class WebRTCWrapper {
     boolean isVideoEnabled() {
         final VideoTrack videoTrack = this.localVideoTrack;
         if (videoTrack == null) {
-            throw new IllegalStateException("Local video track does not exist");
+            return false;
         }
         return videoTrack.enabled();
     }

src/main/res/values/strings.xml

@@ -1137,4 +1137,5 @@
     <string name="record_voice_mail">Record voice message</string>
     <string name="backup_started_message">The backup has been started. You’ll get a notification once it has been completed.</string>
     <string name="error_security_exception">The app you used to share this file did not provide enough permissions.</string>
+    <string name="unable_to_enable_video">Unable to enable video.</string>
 </resources>