Move 'show hidden channels' option

Make it more prominent to make it more clear what's happening
when you unblock an app

Test: DeletedChannelsPreferenceControllerTest
Test: ShowMorePreferenceControllerTest
Flag: com.android.server.notification.notification_hide_unused_channels
Bug: 322536537
Change-Id: I745b2037b4dc907a4307fa7f70ecc3a4c9db2dd2