Bugfix: java.util.ConcurrentModificationException on mPanelSlideListeners

[gfred]

With the lastest version of the AndroidSlidingUpPanel we receiving some crashes with the following exception:

java.util.ConcurrentModificationException
at java.util.ArrayList$ArrayListIterator.next(ArrayList.java:573)
at com.sothree.slidinguppanel.SlidingUpPanelLayout.dispatchOnPanelStateChanged(SourceFile:643)
at com.sothree.slidinguppanel.SlidingUpPanelLayout.setPanelStateInternal(SourceFile:1108)
at com.sothree.slidinguppanel.SlidingUpPanelLayout.access$100(SourceFile:32)
access$602
access$700
access$900
at com.sothree.slidinguppanel.SlidingUpPanelLayout$DragHelperCallback.onViewDragStateChanged(SourceFile:1340)
at com.sothree.slidinguppanel.ViewDragHelper.setDragState(SourceFile:911)
at com.sothree.slidinguppanel.ViewDragHelper$2.run(SourceFile:336)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:6145)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1399)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)

With this pull request, i'll hope to fix this problem. I know it's a not the best solution, but even better than crashing the app :(

[charlesdurham]

http://docs.oracle.com/javase/6/docs/api/java/util/concurrent/CopyOnWriteArrayList.html Seems like a simpler solution.