Fix screenshot not getting cleaned up after recents animation cancelation

Before this change, mCanceledThumbnail is used but never assigned.

Test: Turn on live tile, swipe up to overview from app and observe that the screenshot from recents animation gets properly cleaned up.
Fixes: 143307786
Change-Id: I8fba46324c43df661adf12cd1e5d9e06a0a3ee6f