Use main display assigned to visible background user as fallback display for IME

Previously, the default display would be used as the fallback display where the IME should be shown.
However, a visible background user can be assigned to a secondary display, which is not the default display.
For a visible bg user, the main display assigned to the user should be used as the fallback display.

This changelist was cherry picked from https://partner-android-review.git.corp.google.com/c/platform/frameworks/base/+/3014818 .

Bug: 383228193
Flag: android.view.inputmethod.fallback_display_for_secondary_user_on_secondary_display
Test: atest InputMethodManagerServiceTests
Test: atest CtsWindowManagerDeviceIme
Test: atest --user-type secondary_user_on_secondary_display CtsWindowManagerDeviceIme

Change-Id: I643bedaf9de1ed83d226133293fb4a03e4e52a1f