surfaceflinger: fix a potential child layer leak

We should not remove a child layer from its already removed parent.
Call p->removeChild only after we've checked that the ancestor is
alive.

Apply e6b63e1a and this fix to
SurfaceFlinger_hwc1.cpp as well.

Bug: 37121786
Test: manual stress test
Change-Id: I7b811450a998acc4ad9690bd4eda058ce6588e14