Disallow stashing if a drag mirror is showing.

This is to prevent a PiP window from being stashed while a drag operation
that crosses displays is being performed.

Fixes: 414749818
Test: manual test and atest PipDisplayTransferHandlerTest
Flag: com.android.window.flags.enable_dragging_pip_across_displays
Change-Id: I906a78cd728afe820ea4128f8725eaefb59e1555