Fixed crashes due to HSUM issues

+ moved model conversion logic from UI layer to Domain layer
+ replaced default context (system) with current user context

Fix: 369425977
Test: Manual - open shortcut helper from Chrome on HSUM enabled device,
ensure device doesn't crash
Flag: com.android.systemui.keyboard_shortcut_helper_rewrite

Change-Id: Ifb9f0b6a193953190fa77a0b7865f6131f74a38d