Fix re-registering PositionUpdateListener

Nulls out `info.mPositionUpdateListener` after unregistering the
Listener, allowing a new PositionUpdateListener to be registered later.

`addPositionUpdateListener` was only called if
`info.mPositionUpdateListener` is non-null.
The Listener is first created and registered if the system needs to keep
track of the View's bounds (eg. if it is marked as a keep clear area).
If the system no longer needs to keep track of the View's bounds, the
Listener is unregistered, but wasn't set to `null`.
When the system later needs to receive updates about the View's bounds
again, the listener failed to be re-registered because it was non-null.

Bug: 226583836
Test: Manual, check position updates of View
Change-Id: Ia494e84843a9954e368d76a000f5f725e3a58df1