Upgrade OkHttp to 3.14.x

https://square.github.io/okhttp/changelog_3x/#version-3130

OkHttp 3.12.x will receive critical bug fixes until the end of 2021. Newer versions drop support for Android 4, so when we upgrade we'll either need to drop support for Android 4 or restrict features that use HTTP (RSS feeds, mailbox) to Android 5+.

According to Google Play, 0.5% of devices running Briar use Android 4.

Depends on #2221.