Let insets sources can produce insets on 0-width or 0-height windows

Window Manager allows the client to add 0-width or 0-height windows.
These windows can get insets in the legacy insets mode even they only
intersect with the insets source window on the edges. This CL make this
behavior compatible with the legacy insets mode.

Fix: 150696052
Test: atest InsetsSourceTest
Change-Id: I9ed76bb615a0133ad55e1f93b22fbc03ae5cb437