Fixed Peristent search in shortcut helper

Previously when a search query was typed in shortcut helper, this query
would persist even after shortcut helper was closed and reopened.

This fixes the issue, resetting the search query onViewClosed.

Test: ShortcutHelperViewModelTest
Flag: com.android.systemui.keyboard_shortcut_helper_rewrite
Fix: 381244038
Change-Id: Id5ff5381b40d40181a45e5717283687b757312c5