Nullify mParallelRunningAnim after it ends to avoid updating it

Keeping the stale reference after the animation ends is dangerous because if we call end() on it, it will jump to the now-obsolete end value.

Test: atest NexusLauncherTests:com.android.launcher3.memory.MemoryTests#testAppLaunchFromWorkspace
Bug: 193440212
Change-Id: I8ca45db7ceec2d6e6a9c6e69a6e0359fd73a7d27