trust user-installed CAs for self-signed servers

Port the Flutter app's network security config: add a base-config that trusts system and user certificate anchors and permits cleartext, and reference it from the application manifest. android:usesCleartextTraffic is now redundant and removed.

app/src/main/AndroidManifest.xml

@@ -15,10 +15,10 @@
         android:enableOnBackInvokedCallback="false"
         android:icon="@mipmap/ic_launcher"
         android:label="@string/app_name"
+        android:networkSecurityConfig="@xml/network_security_config"
         android:roundIcon="@mipmap/ic_launcher_round"
         android:supportsRtl="true"
-        android:theme="@style/Theme.Subtracks"
-        android:usesCleartextTraffic="true">
+        android:theme="@style/Theme.Subtracks">
 
         <activity
             android:name=".MainActivity"

app/src/main/res/xml/network_security_config.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="utf-8"?>
+<network-security-config>
+    <base-config cleartextTrafficPermitted="true">
+        <trust-anchors>
+            <certificates src="system" />
+            <certificates src="user" />
+        </trust-anchors>
+    </base-config>
+</network-security-config>