Check that onAttach() is called before onResume() to prevent NPE.

Sometimes, the context is null in TutorialController which leads to an
NPE when relying on context specific functions. This change checks that
context is null and if so, defers the context specific functions that are
called until after onAttach() is called.

Fix: 307781152
Test: Set context to null in onResume() and check that the tutorial
continues like normal
Flag: None

Change-Id: I66e345f8361315787534bb88b9c1d12278708d78