Bump to avs 5.0.18 fixing couple crashing cases (#2146)

Author: Chien Pang Lee

app/build.gradle

@@ -31,7 +31,7 @@ ext {
     avsGroup = 'com.wire'
 
     // proprietary avs artifact configuration
-    customAvsVersion = '5.0.17@aar'
+    customAvsVersion = '5.0.18@aar'
     customAvsInternalVersion = customAvsVersion
     customAvsName = 'avs'
     customAvsGroup = 'com.wearezeta.avs'