Destroy docked divider surface when it's hidden.

Also includes bunch of small refactorings:
* destroying surfaces is now fully contained within
WindowManagerServices and mDestroySurface can be privatized;
* WMS.isDockedStackResizingLocked can be removed;
* mScreenCaptureDisabled changes from being SparseArray<Boolean> to
SparseBooleanArray, which not only avoids boxing but also makes code
simpler (no need to check for null)

Bug: 25844096
Change-Id: I0e5462760ffbc947ce6dc52ef429fa270ffc6786