Log terrible failure on null PiP task token

Log an extra terrible failure for the null PiP task token in
PipController#onDisplayChange. We are not able to reproduce the original
NPE, adding extra log here to help further investigation.

Flag: com.android.wm.shell.enable_pip2
Bug: 378743431
Test: N/A log-only
Change-Id: Iacf3714e7cd201bdaac44a379049119d2c343dc6