android: Extend GUI so the MTU can be entered

android: Extend GUI so the MTU can be entered

This also adds a new area for advanced settings that is only displayed
if the user requests it (or if advanced settings already have been set).

The min. MTU for IPv6 is 1280, anything lower lets the TUN device
creation fail if an IPv6 address has been assigned.  If lower MTUs are
necessary we might be able to catch that later when setting the MTU and
just use at least 1280 if an IPv6 address was assigned, but let's keep
it simple for now.