Move "Close window" shortcut to Multitasking category

To refrain from accidentally closing tasks which should not be closed by
users, the close shortcut is changed to close only desktop tasks.

This change moves the "Close current window" shortcut from the
"System" category to the "Multitasking" category in the shortcut
helper to reflect the change.

Bug: 416437920
Test: MultitaskingShortcutsSourceTest
Flag: com.android.window.flags.close_task_keyboard_shortcut
Change-Id: I874c9ed850d830356ad1aa68428e9fd094e177a3