Allow custom DNS on opportunistic DoT

[aitorvs]

Task/Issue URL: https://app.asana.com/0/1202552961248957/1207692081807518/f

Description

Remove the warning on custom DNS screen when private DNS is set to automatic

Steps to test this PR

Test automatic mode

• [ ] install from this branch
• [ ] ensure private DNS is set to off
• [ ] enable VPN and set VPN custom DNS to 1.1.1.1
• [ ] verify custom DNS is correctly enabled
• [ ] go android settings -> private DNS and set to dns.google
• [ ] back to DDG app -> VPN custom DNS screen
• [ ] verify warning "private DNS is already specified...turn off private DNS..." shows
• [ ] verify internet connection works
• [ ] verify DNS traffic is routed through Google (dnsleaktest.com)
• [ ] go android settings -> private DNS and set to automatic
• [ ] back to DDG app, VPN custom DNS screen
• [ ] verify warning "private DNS is already specified...turn off private DNS..." does NOT show
• [ ] verify internet connection works
• [ ] verify DNS traffic is routed through Cloudflare (dnsleaktest.com)

[aitorvs]

• #4703 👈
• develop

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