VPN-6629: Fix debug daemon timer on Android

[mcleinman]

Description

There is a debug setting for short session timers - sending pings every 3 minutes when a VPN session is active, rather than the typical every 3 hours.

History of this:

• It was initially written as a lazy var, which caused a crash in some situations. (VPN-6629)
• It was then updated to not be lazy, but that caused the value to always default to false - as the mConfig wasn't saved when the computed var was trying to use it. (VPN-5766 - Fix CountdownTimer generation timing #8555)

In this PR, I change the timer to use onTick for debug timer pings. I had initially tried to just move the timer setup work to right before the timer is started, as then we'd have mConfig already set. However, that requires being on a UI thread (it's a CountDownTimer thing). I'm confident there is a way to do this, but I'm not an expert on Android - and I spent a while testing stuff out to get this UI thread from a background daemon, getting nothing working. This alternative way of handling it should suffice, and tests okay for both the debug option being active and not active.

Reference

VPN-6629

Checklist

• My code follows the style guidelines for this project
• I have not added any packages that contain high risk or unknown licenses (GPL, LGPL, MPL, etc. consult with DevOps if in question)
• I have performed a self review of my own code
• I have commented my code PARTICULARLY in hard to understand areas
• I have added thorough tests where needed

[lesleyjanenorton]

👍