Arrow down navigates from search bar in shortcut helper

Fixes: 363257904
Flag: com.android.systemui.keyboard_shortcut_helper_rewrite
Test: Manual - open shortcut helper, press down while focus is on the
search bar, ensure focus moves to the next focusable element down.

Change-Id: I5d4ce4536e5a7f8f2f6b8bcc5f3d21a08b14bece