Fix no correct window observer of an embedded display

The window observer of an embedded display could not be set to the
window observer of its parent display if the activity on it launched
before A11y service on. Doing this mapping to make sure the behavior
on embedded display could be correct when this situation happened.

Bug: 143113272
Test: a11y CTS & unit tests
Test: manual testing
Change-Id: Ic00a752db421179d80439662f69d0cfa1f28254a