Remove drag mirrors on EXITED_PIP.

This ensures that the mirrors get removed when pip is exited, for
example, while dragging PiP, open the same app to go from PiP > full
screen.

Fixes: 408982524
Flag: com.android.window.flags.enable_dragging_pip_across_displays
Test: atest PipDisplayTransferHandlerTest
Change-Id: Ie4d2a22e52db61607056f648c7ea922e21741ee5