surfaceflinger: fix layer count

When a layer is removed, mLayerCount is always decremented by 1 even
though the layer has children.  This commit fixes that by making
sure mLayerCount is properly decremented, and makes sure that a
parent layer must be a valid layer in mCurrentState.

Bug: 37121786
Test: manually open and close Camera app
Change-Id: I5a3e18be41646b167b4d361dfe08db4d06e1c2d0
(cherry picked from commit 98f1c108)