Show instructions for locking Briar to the Xiaomi recent apps list

This branch adds a button to the setup screen showing instructions for locking Briar to the recent apps list on Xiaomi/Redmi phones.

The ability to lock apps to the recent apps list has existed since MIUI 6 (released in 2014) and perhaps earlier. Since MIUI 10, locking is done by long-pressing on the app rather that swiping down, so the instructions depend on the MIUI version, which is independent from the Android version and has to be looked up using a system property.

I've only tested this branch on one phone, the Redmi Note 7 (MIUI 12). It's possible that the system property holding the MIUI version is missing or has a different format on other devices. If we can't parse the system property we default to showing the instructions for MIUI < 10.

It's possible that some users are running MIUI < 6 on Android 4. I haven't been able to find out whether the lock feature exists on MIUI < 6. Checking the system property for a version < 6 doesn't seem very useful, as we don't know if the system property exists on those versions. So users running those versions may see instructions that don't apply to their device, but they can still continue with setup.

Closes #2009 (closed)