Capturing user's selected custom key combination for shortcuts

When user presses a key combination we capture it, and display on the
UI.

Test: ShortcutCustomizationViewModelTest
Flag: com.android.systemui.keyboard_shortcut_helper_shortcut_customizer
Bug: 373631227
Change-Id: Iac6718b5377108849e915ea8121be7a8da44c890