Reland "Fixed constant window switching on lock screen..."

This is a manual reland of I3680256d41793f62def42fda00e26db1dcc9,
which was certainly merged into lmp-mr1-dev then auto-merged into
master but silently lost there for unknown reasons when
I8c42a1e6091b0fe1253e90265ac248087e was auto-merged into master.

Changes in WindowAnimator.java was already covered by
I2933eaf0ab55fe31cb382c46c411033e33a756e0 so this CL does not
include that change.

Bug: 18021493
Bug: 22158649
Change-Id: Ib1bf9f5bef44d0400230afc32231f7d1f3a1c98b