AudioFlinger: Use std::any for the event cookie

This generalizes over the previous wp<RefBase> cookie.

Test: atest mediasyncevent_tests
Test: atest synchronizedrecordstate_tests
Test: atest MediaSyncEventTest
Bug: 288339104
Bug: 288943829
Merged-In: Iaa535450550681517613310a7043526be38c2aae
Change-Id: Iaa535450550681517613310a7043526be38c2aae
(cherry picked from commit 8f5bb22b)