Protect against assumptions of an existing home stack.

Since there is no longer a guarantee that a home stack will
exist this:
- Adds a method to create a home stack if the display should have one
  and it doesn't exist.
- Replaces calls to DisplayContent#getHomeStack() if those calls assumed
  a home stack would exist.

Bug: 144085050
Fixes: 144085050
Test: RootActivityContainerTests
Test: DisplayContentTests

Change-Id: Iec6b5ccf66fed7777d19b61c5624bd097edb6786