Added UI for displaying existing custom shortcuts

+ Added button for deleting custom shortcuts commands if they exist for
  a particular shortcut

Test: manual - ensure custom shortcuts UI corresponds with UX mocks
Test: ShortcutCustomizationViewModelTest
Flag: com.android.systemui.keyboard_shortcut_helper_shortcut_customizer
Bug: 373620793
Change-Id: Ifa76e379959fbf6fcf332d99a520acf156770a2b