Camera: drop stale callback buffer

Sometimes, when preview size is changed and preview callback is enabled, stale
callback buffer from previous size could be delivered to callback processor
because stop preview doesn't guarantee consumer side buffers are properly
cleaned up. We need drop these buffers to avoid sending wrong data to app side.

Bug: 11009183
Change-Id: If9281e8c02481f883872ce58dfd8660a06d56a47