[PiP on CD] Fix PiP dismiss target on external display.

This CL implements getting the correct WindowManager for the associated
display where PiP is active so that the PiP dismiss target shows up in
the correct display.

Bug: 391699824
Test: Manually check that the dismiss target shows up on the correct
display when dragging PiP on the external display
Flag: com.android.window.flags.enable_connected_displays_pip

Change-Id: I08615d90e3f6cf21697f6cbd66effba94377ca3e