Don't allow 0-sized views to be focused.

These zero-sized views tend to make keyboard navigation
difficult for users since focus "disappears" sometimes.

This takes a best-effort approach for focus requested
prior to layout: If a View hasn't been laid-out and is
asked to take focus, it will act as though this size
constraint doesn't exist. Then, upon layout, it will
defocus itself if it still has no size.

Bug: 32072305
Test: Added CTS View_FocusHandlingTest#testSizeHandling
Change-Id: I33977247272f4551f3f095680867fd8d1e30682b