SurfaceFlinger: Select which layer state to visit

Modifies the traverseIn[Reverse]ZOrder methods to also take an enum
value specifying whether to traverse the current state or the drawing
state.

This has the effect of fixing a bug where we weren't performing
transactions on a child layer because its parent was only visiting its
drawing layers (rather than its current layers) and was thus skipping
the child, which had not yet been moved from current to drawing.

Bug: 36858924
Test: ChildLayerTest.Bug36858924 doesn't hang
Change-Id: I1959f40bc07e77864ba024511d429592a398a67a