Select the focused display as fallback only if the display can host tasks

Otherwise, still select the default display.

It is possible for a display that cannot host tasks becomes focused
if there's a focused window on the display.

Bug: 406967985
Test: MultiDisplayActivityLaunchTests
Flag: com.android.window.flags.fallback_to_focused_display
Change-Id: I8b498ac4fe2a291ad8334f30b2f023c7c76724a8