Retain dimming while animating and obscured.

Fixes for two dimming problems.

- Dimming was turning off at the start of animation because it was
dependent on hiddenRequested which becomes true while the view is
still visible.

- Looking for windows with FLAG_DIM_BEHIND set stopped at the first
obscured window. But Z-order shuffling due to Animation.setZAdjustment
causes the window list order to be different than the display order.
Consequently windows that are being shown are declared obscured by
handleNotObscuredLocked().

Bug 7396404 fixed.

Change-Id: Ic59150964d3950e29b115da5f6c0f07a64190d44