Switching the accessibility poolable classes to the new pool management APIs.

The pool management APIs were simplified and using them requires much less
code than implementing your own pooling. Using common pooling logic is
also less error prone. This change swithces AccessibilityEvent and
AccessibilityNodeInfo to the new APIs.

Change-Id: I2dcfe2e1b0b0be5f89bd92359766ae3d6f8a143b