Remove activity from stopping list when removed from display

Organized tasks and its children were removed when systemui died.
And the activities should no longer waiting to be stopped.

Also prevent NPE if the transaction is applied to a unknown WC.

Bug: 170451499
Test: presubmit tests
Change-Id: I2fcef2ffc8c962c0dac3b56cdbea4f17e071152d
2 files changed