Can't use HTTP proxies requiring credentials

[jdm-square]

Describe the bug
Flank supports using an HTTP proxy by setting the http.proxyHost and http.proxyPort properties. This can be demonstrated by installing mitmproxy and setting the properties to point at localhost and 8080; the connection will fail because of certificate pinning, but it's proof that the correct network connections are happening.

However, if mitmproxy is run with --proxyauth user:pass to require a username of user and password of path, Flank hangs.

To Reproduce

Steps to reproduce the behavior:

1. Install mitmproxy
2. Run mitmproxy --proxyauth user:pass
3. Run java -Dhttp.proxyHost=localhost -Dhttp.proxyPort=8080 -Dhttp.proxyUser=user -Dhttp.proxyPassword=pass -jar ./test_runner/build/libs/flank.jar firebase test android run

Expected behavior
Flank's behaviour with an authenticating proxy should be identical to its behaviour with a non-authenticating one.

Details (please complete the following information):

Have you tested on the latest Flank snapshot?

Post the output of flank --version.

version: local_snapshot
revision: 77c80103e61ee101d0de145f8ba1390216993665
session id: f6cd4a9e-d9ab-44df-90eb-ff44e7528444

Additional context
Related to #1464 and #776.