Describe the bug
If a custom DNS-over-TLS ("Private DNS") server is configured, enabling tailscale will stop all DNS requests and show a "couldn't connect" in the android settings app.

To Reproduce
Steps to reproduce the behavior:

1. Disable the tailscale VPN.
2. In Android setting's "WiFi and network" section, set the private dns to a specific server, not "automatic". I tested with values of both "dns.quad9.net" and "dns.nextdns.io".
3. Observe that dns works as expected.
4. Activate tailscale.
5. Observe that dns requests fail, and that private dns' settings are labelled "couldn't connect"

Note that the "automatic" setting, for Google's DNS-over-TLS server, works correctly.

Expected behavior
DNS-over-TLS servers should be accessible even when running tailscale.

Screenshots

Version information:

• Device: OnePlus Phone
• OS: Android
• OS version: 10.
• Tailscale version: version 1.2.2-t76c2982d8-g39dfd8495

Additional context