Make DisplayContent multi-display and multi-user aware for IME

This changelist updates the DisplayContent IME related methods
(getImeHostOrFallback and computeImeControlTarget) to correctly
determine the user's main display for visible background users.

The main display for a visible background user might not be DEFAULT_DISPLAY,
therefore the IME fallback display has to be user's main display instead (which
will be other than DEFAULT_DISPLAY).

This modification is a necessary prerequisite to fix the IME logic for
identifying the appropriate fallback display for IME visualization.

Flag: android.view.inputmethod.fallback_display_for_secondary_user_on_secondary_display
Test: atest CtsWindowManagerDeviceIme CtsInputMethodTestCases
Test: presubmit
Bug: 383228193
Change-Id: I5f85149684b9b1c6db13bed8b6138d67b4eb2ae1