Switch global key registry to be owned by the build owner. (#74701)

This gets away from the extra static map for global key registrations
in favor of the data structures being instance properties of the build
owner. This still allows for semantically-equivalent static access through
the binding (which in turn gives access to the build owner).

This also adds a `BuildOwner.globalKeyCount` getter to get the count
of global keys associated with widgets currently in the tree.