GraphicsTracker: Do not crash from getting AHardwareBuffer id

In certain cases a GraphicBlock not based on AHardwareBuffer can be
passed to GraphicsTacker. Currently those blocks cause crashes on
getting AHardwareBuffer id. Do not crash in those cases.

Bug: 362729602
Flags: EXEMPT bugfix
Test: m
Change-Id: I347a8da090f75c5b68901cf225d74a2d0bb72b7c