libaurum: Update accessibility event handling to use shared pointers

libaurum: Update accessibility event handling to use shared pointers

Changed the EventHandler typedef to use std::function with a shared pointer to AccessibleNode instead of a string package name.
Updated the A11yEventHandler operator() to accept a shared pointer to AccessibleNode instead of a string package name.
Updated the AtspiAccessibleWatcher processCallback method to accept an optional AtspiAccessible pointer and create a shared pointer to AtspiAccessibleNode when available.

Change-Id: I4911407a203a73c3c65d80fbc3843ac8762d9903