Review the VpnService.setUnderlyingNetworks code

[ameshkov]

First of all, we can use newer APIs on P. Look at what's done in Shadowsocks.
shadowsocks/shadowsocks-android@0af4500

Second, there are some Xiaomi-specific workarounds, we should check if we still need them.

Original issue: #1587

Btw, OpenVPN simply sets it to null:
https://github.com/schwabe/ics-openvpn/blob/450e99c135ebabbf911d6880e39811d88454cdd1/main/src/main/java/de/blinkt/openvpn/core/OpenVPNService.java#L879

[sviperz]

The issue is easy reproducible on the latest Android 10 release and AG Nightly. You just take a new photo and watch Google Photos waiting for WiFi infinitely though it is definitely turned on.

Please escalate as since backup using mobile data is usually turned off, the photos are never back up to the cloud when AG is switched on.

Environment

• Adguard version: v3.3. nightly 7 (50)
• Adguard filtering mode: VPN
• Device model: Google Pixel 3
• OS: Android 10
• Google Photos version: 4.25.0.271222815
• No root

[artemiv4nov]

@Revertron you lead this task.
@admitrevskiy you need to support ur teammate, checking the decision on Xiaomi devices.