Changing the fallback behavior to launch on focused display

The original fallback behavior was to launch on the default display,
which seems not making sense anymore in a multi-display scenario.

Use the focused display instead as it is likely what the user
currently focus on.

Bug: 406967985
Test: MultiDisplayActivityLaunchTests
Flag: com.android.window.flags.fallback_to_focused_display

Change-Id: I9b63ece4a3c2728b19d6c25b25b518e8f36b1990