Clear task snapshot when we showSurfaceBehindKeyguard.

To prevent extra blocking calls to ActivityManager,
rather than calling
ActivityManagerWrapper#invalidateHomeTaskSnapshot,
add a transit flag to have ActivityTaskManager
handle this as part of the goingAway call.

Bug: 229890190
Test: atest SystemUITests
Change-Id: Ica1b552a332d0c946d6008965c1a2881b646e365