Ensure that finish() means no more screenshots are taken

This fixes broken animated gifs I've seen where an additional image was
written on after the gif was 'finished'.

This was basically a race condition between the log-listener thread and
the main thread - the main thread will now block while waiting for
the last screenshot to be taken if it's already in progress, which works
much better.

paparazzo/src/main/java/com/github/rtyley/android/screenshot/paparazzo/OnDemandScreenshotService.java

@@ -50,7 +50,7 @@ public void start() {
      */
     public void finish() {
         if (logListenerCommandShell != null) {
-            logListenerCommandShell.cancelled = true;
+            logListenerCommandShell.cancel();
         }
         for (ScreenshotProcessor screenshotProcessor : processors) {
             screenshotProcessor.finish();
@@ -86,13 +86,17 @@ public void activate() {
             activated = true;
         }
 
+        public synchronized void cancel() {
+            cancelled = true;
+        }
+
         @Override
         public boolean isCancelled() {
             return cancelled;
         }
 
         @Override
-        public void processNewLines(String[] lines) {
+        public synchronized void processNewLines(String[] lines) {
             if (!activated || cancelled) {
                 // log.debug("Ignoring "+lines+" lines of log");
                 return;