Destroy dep tree on RecentsView detached. This prevents leaks.

Scenario 1 was already working, this CL fixes scenario 2 and leaves scenario 3 broken.

3 Scenarios:
Scenario 1 (occurs in real usage):
Launcher instance 1 init
Launcher instance 2 init
Launcher instance 1 destroy
Launcher instance 2 destroy

Scenario 2 (occurs in testStressPressHome):
Launcher instance 1 init
Launcher instance 1 destroy
Launcher instance 2 init
Launcher instance 2 destroy

Scenario 3 (does not currently happen):
Launcher instance 1 init
Launcher instance 2 init
Launcher instance 2 destroy
Launcher instance 1 destroy

Fix: 381160061
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Test: TaplStartLauncherViaGestureTests#testStressPressHome
Change-Id: I7c82ce97e7a7f0698b9adad38f59e4555f0d1bdc