Cleanup PreparationCoordinator a bit

Prelude to adding new filtering functionality.

Mostly minor fixes -- only significant change is that we get rid of some
redundant calls to mInflatingNotifs.remove(). The abortInflation()
method should really be handling those.

Test: atest SystemUITests
Change-Id: Iea4006140979c0c10f4a857aa5ca5bd9dacc22d6