graphics: keep mapper valid during process termination

GraphicBufferMapper is valid during process termination.  IMapper
must stay valid as well.  It should not rely on global/static
objects that may have been destructed.

Bug: 37480313
Test: libgui_test
Change-Id: Icb8079153306f2465c26c0f1ce812895ad83f21b
2 files changed