[fix] Picker default font styles

[paularmstrong]

Problem: Picker was using Times as the font-style, because it was using inherit.

Solution: Same as applied for TextInput in b28cbbb, set fontFamily to System

[PeterKottas]

Hi Paul, just a quick question as it seems you understand what you did there ;) . What exactly will system mean in this context? I am changing the fontFamily for all text inputs via overriding default props which works fine for me. However, picker is not picking this up and I have the feeling that this PR might fix it. I am just not totally sure if I am right. Will this somehow enable me to change font family for picker? Ty

[PeterKottas]

Ah sorry, I just figured it out. My problem was typescript typings are incorrectly forbidding one from passing fontFamily, color, or fontSize. That's why I assumed it's unsupported but when I forced the values in, it all started working. Also in the light of this, the PR makes much more sense now ;)

[PeterKottas]

Weirdly I get an error when doing this:

   Warning: Failed prop type: Invalid props.style key `fontFamily` supplied to `Picker`.

But this rly has no longer anything in common with this PR so I'll open separate issue. I just feel there's definitely some weirdness around picker.