Improve accessibility on notification on ls settings page

Improve accessibility on notification on ls settings page by making
the minimalism setting buttons not focusable when already selected.
This change reduces the confuses and improve usability for both
talkback and voice access.

Fix: 415946053
Fix: 415944893
Test: manual in the Settings App
Flag: com.android.server.notification.notification_lock_screen_settings
Change-Id: I339c09480440d705153373630e84b2f4fbc1330c