Avoids an implicit downcast + crash in MouseTracker, since the last
pointer event can be a Move or Down event, in addition to Hover.

* Fix MouseTracker annotation leak

Map's remove method is typed `remove(Object key)`, which can cause bugs.

Modified the existing test to check if the annotation has been removed
from MouseTracker.

This implements support for hovering mouse pointers, so that mice connected to Android devices, and ChromeOS devices running Android apps will work properly.

It teaches flutter_test about hover events, which required changing how they are created and used.

Also modifies AnnotatedRegion to allow a region that can be located someplace other than just the origin.

Along with tests for all of the above.

Fixes #5504