Accept user CA's for connections via HTTPS proxy

Permit Traccar client to communicate with the Traccar server via an
HTTPS proxy that uses a certificate generated from a user (non-system)
CA.

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

@@ -1,4 +1,9 @@
 <?xml version="1.0" encoding="utf-8"?>
 <network-security-config>
-    <base-config cleartextTrafficPermitted="true" />
+    <base-config cleartextTrafficPermitted="true">
+        <trust-anchors>
+            <certificates src="system" />
+            <certificates src="user" />
+        </trust-anchors>
+    </base-config>
 </network-security-config>