[HWUI] Add null check for CanvasContext

If ReliableSurface is forced to acquire a fallback buffer then the
Surface may be abandoned. When getting frame timestamps we need to check
that the Surface still exists.

Bug: 152262035
Test: builds, boots
Test: dumpsys gfxinfo
Change-Id: Ifdb198ebf74cc9dc681c4ab51c4901176a7f5fc9