Make ModifierShortcutManager multiuser aware.

Instead of using the cached system user context, we now keep
track of the current foreground user (maintained by PhoneWindowManager)
and use that user to resolve Roles and PackageManager queries. This
means that we will resolve the correct application launch intents for
the appropriate user.

This is especially important in HSUM builds, where the system user is
never the foreground user and so roles would never resolve correctly.

Flag: com.android.server.flags.modifier_shortcut_manager_multiuser
Bug: 351963350
Test: atest ModifierShortcutManagerTests ModifierShortcutTests

Change-Id: I98d70ebd1f067d27df52275bf82517fab1473ec8