Previously we polled every 2.5 minutes to get the fx spot price,
and had a 10 minute cache expiry during which the latest spot price
was valid.
On Android, this often resulted in having no price available (showing
"No data" in GUI) when putting the app in the foreground after e.g.
a half-hour sleep in the background: often there would be no fx price
until the next tick, which could take 2.5 minutes. (btw in some cases
I saw the application trying to get new quotes from the network as
soon as the app was put in the foreground but it seems those happened
so fast that the network was not ready yet and DNS lookups failed)
Now we make the behaviour a bit more complex: we still fetch the price
every 2.5 mins, and the cache is still valid for 10 mins, however if
the last price is >7.5 mins old, we become more aggressive and go into
an exponential backoff, initially trying a request every few seconds.
For the Android scenario, this means there might be "No data" for fx
for a few seconds after a long sleep, however if there is a working
network, it should soon get a fresh fx spot price quote.
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