Initialize a default log level

Clean up has been broken by 3df63c579d.

The verbosity was correctly initialized for the Server process, but not
for the CleanUp process.

To avoid the problem, initialize a default log level.

server/src/main/java/com/genymobile/scrcpy/Ln.java

@@ -15,7 +15,7 @@ public final class Ln {
         DEBUG, INFO, WARN, ERROR
     }
 
-    private static Level threshold;
+    private static Level threshold = Level.INFO;
 
     private Ln() {
         // not instantiable