Add a listener to know when the list of root views changes.

Add a listener to WindowInspector that notifies listeners when the list
of root views has changed.

Flag: android.view.flags.root_view_changed_listener
Bug: 394397033
Test: atest CtsViewTestCases
Change-Id: I65e09d74a2e3c0896bb5ea222813311dd20d622e