Ensure visible regions are recomputed when children change.

Currently a reparent operation by itself may not be enough
to trigger visible region computation, leading to invalid results
on-screen. Note that reparent does not change the geometry, or the state
sequence number, this means that doTransaction does not return Layer::eVisibleRegion.
For layer addition and removal we explicitly mark the visible regions dirty (see
mLayersAdded and mLayersRemoved in SurfaceFlinger.cpp). We implement a similar model for children,
with parents setting mChildrenChanged, and consuming it in doTransaction in order
to emit an eVisibleRegion flag.

Test: Manual
Bug: 123333167
Bug: 123131546
Bug: 123285451
Change-Id: I131c814e6bde927353f78c7f62d98b0f5cd2ff73