Do not verify display context in DecorContext constructor

Non-activity context can be passed to DecorContext, so an exception
can be thrown when Context#getDisplay() call does verification.
An example is that a free dialog uses Application context as its context.

Bug: 149928768
Test: manual: test crashed apps reported in bugs before/after this patch.
Change-Id: Ib302c23cb8e2e388e7468f37f1b63b83ef52ef0e