Unify syntax for variables in CSV inputs

[sarathms]

We use react-intl in our web projects which expects source strings to use {} to declare variables. e.g "This is a {variable}"

For mobile projects, we follow a slightly different syntax ("This is a {{variable}}"). This prevents us from reusing strings from the mobile projects in say probe-desktop where we want to achieve very close resemblance.

If the syntax used the mobile copy, isn't coupled with a 3rd party dependency package, maybe we can start using the {variable} format for all projects across the board. I am guessing this just needs changes to the json-to-android-xml.py script.

cc: @hellais @lorenzoPrimi

[lorenzoPrimi]

Native Android and iOS app can't use {variable} natively, there can be a third part library but I won't rely on that.