Stuck "shutting down" if system private DNS is enabled but is not reachable

[Mygod]

To reproduce this bug:

1. Use Android 9 or Q beta (master branch required as Enhancements for Android Q beta #2161 should have fixed this for Android Q by using new non-blocking APIs, but still reproducible on Android 9); (I have not tried to reproduce this on Android Q yet)
2. Enable private DNS (set it to dns.google or something);
3. Wait for system to verify the private DNS is working (it should show dns.google in system settings below "Private DNS");
4. Simulate network instability using su -c 'iptables -I INPUT -p tcp --sport 853 -j DROP' (drops all incoming DNS over TLS traffic);
5. Enable Shadowsocks VPN with route set to bypass China; (so that local DNS resolving is enabled)
6. Tap test Internet connectivity a lot of times (>64 times to be safe so that IO thread pool would be full);
7. Attempt to shut down service. (if the bug did not occur, go back to 5)

What happened: All IO threads are blocked at native call android_getaddrinfo, specifically, fread, which never returns.

Curiously, this does not happen for regular DNS traffic as there seems to be timeouts for those, but not for DNS for TLS.

Reported this to AOSP: https://issuetracker.google.com/issues/133874590