Fix threading issue with splash screens

If we use a different thread to remove the view than we added it,
it leads to suffering. WindowManagerGlobal.removeView ends up
setting mParent of the DecorView to null, without any locking,
which can lead to crashes.

Test: Device boots, starting windows work. Otherwise: Pray
Change-Id: Ifc1e0d0236746aef4ed2b63e08af7de695ce69aa
Fixes: 63664477