Add mode: Choose name and icon for new custom modes

This also unifies the "icon picker" screen with the new "add mode" screen since in both cases we want to edit name and icon together (and not save updates until the user confirms).

Bug: 326442408
Bug: 346278854
Test: atest com.android.settings.notification.modes
Flag: android.app.modes_ui
Change-Id: I8a9d07ba0b6c55f3abc1b9884f278d51d178dc83