Don't reparent task surface if it is not valid

If task was closed while converting to bubble, the surface is already released. Make sure we don't attempt to reparent it.

Bug: 401033122
Test: drag bubble to fullscreen
Flag: com.android.wm.shell.enable_bubble_to_fullscreen
Change-Id: I5f2e432c7b3a1ff22225777c719e350517ea4364