Don't save UI executor in UiListener.

When an activity is killed by the system, a new UiListener is created and attached, but it doesn't have its executor set. Rather than save it as an instance field, just get the Ui executor when needed, by passing the context to the listen method.

Bug: 70510707
Test: unit and manual via "don't keep activities" developer option
PiperOrigin-RevId: 178668338
Change-Id: I5360b525377edab5f3a117d1f0f50bf6da6a6f0c