Remove task thumbnail from cache when receiving onRecentTaskRemoved(1/2)

This is necessary because the original task is removed when launching a
new task.

Bug: 432199332
Flag: EXEMPT Bugfix
Test: dump heap memory after launch new task, verify the thumbnail of
removed task will be removed.

Change-Id: I122a1e4c7eaefec9d1c7c1eb0a05d728f6f9e064