Don't set rounded corners for letterboxed bubbles.

This better matches UX specs, preventing UI elements in fixed landscaped games from getting cropped. This also prevents a surface bug where rounded corners may be punched out on initial load.

Before: screenshot/AU2eZLj9wqkjx9o
After: screenshot/BsnaQkbyjax4HaZ
Fix: 430327866
Test: wm presubmit
Flag: com.android.wm.shell.enable_create_any_bubble
Change-Id: Ic6fd8b27d4e135919d597be36115eb9c19ba1e57