Add snooze ImageView and symbol to core

Part 1 of snooze button guts setup with no visual effects
- core layout - add snooze_button ImageView
- core symbol - add `snooze_button` to core symbols
    for sysui reference via com.android.internal.R.id

Bug: 171817112
Test: manual, no regressions
Change-Id: Ie57d9f156750cecc6c48f8f905cab715142bd27d