Fix setLaunchBehind target being stop while back gesture animating

The home activity will addToStopping when launching another app, if
the back gesture start before home activity stopped, it can still being
stopped and become invisible even set mLaunchTaskBehind = true.

Bug: 131727607
Test: launch an app, and start back gesture before home activity stopped,
monitor the home activity won't become invisible.

Change-Id: I189bc31a5dc55c91c48a1dc81e1336aa580e90c6