SurfaceFlinger: Forward pending buffer release to children.

When releasing the pending buffer because we are removed from the current
state we also need to do so for the children (quite simply, as they are
also removed from the current state).

Bug: 62536731
Bug: 111373437
Bug: 111297488
Test: Transaction_test.cpp
Change-Id: I9a5675241f9980b1d00c19643197f9c1e31cd5f4