[2/n] Limit system fullscreen override to default display

Fullscreen override applied to apps on close to square devices will not
work as well when moved to external displays. This is due to internal
testing which only considered the current internal displays that have
smalller UI changes on task resize.

Limit the current system full-screen override to apply only to the
default display.

Flag: com.android.window.flags.limit_system_fullscreen_override_to_default_display
Bug: 423802240
Test: atest AppCompatAspectRatioOverridesTest
Change-Id: Iec832f9f7f44e547d66602f0d350f32cc97088cb