Introduce feature flag for fallback display for secondary user on secondary display

The IME fallback mechanism is used when the soft input keyboard is expected to appear on a display separate from the one showing the app that is consuming the key input events.

For example, if an app runs on a virtual display created with the DISPLAY_IME_POLICY_FALLBACK_DISPLAY policy, its soft input keyboard will be shown on a designated fallback display, which is currently hardcoded as the DEFAULT_DISPLAY. But that doesn’t work with visible background users. Visible background users have a secondary display other than DEFAULT_DISPLAY as their main display.

Running CtsWindowManagerDeviceIme:MultiDisplayImeTests#testCrossDisplayBasicImeOperations for visible background user is the easier way to reproduce this bug.

Command line to run testCrossDisplayBasicImeOperations for visible background user:
```
atest --user-type secondary_user_on_secondary_display CtsWindowManagerDeviceIme:MultiDisplayImeTests#testCrossDisplayBasicImeOperations
```

Bug: 383228193
Test: m
Flag: android.view.inputmethod.fallback_display_for_secondary_user_on_secondary_display
Change-Id: I283cd17ce962de5d9c1975d98921f60d7eeb203b