Fix some destroy bugs

 Bug: 6961494
 Don't call clearHelpers in destroy(). It is already called
 in onDetach, which is where it needs to be. It can't be
 called by the finalizer

 Switch onCancel listener to onDismiss listener. Fixes an issue
 where destroying can live the webcore thread hung because onCancel
 won't be called, but onDismiss is

Change-Id: I669c389c8c9a81d92c27f88d83bcb711c6c0a5ee