Solutions for our various layout woes

[brentvatne]

These values can update in response to orientation changes or on other events.

• Status bar: translucent or opaque? in phone call? other?
• "Safe areas": iPhone X is the main driver for this at the moment but probably other devices will add other weird layouts in the future.
• Header height and tab bar height can change on layout.
• We may use header height, status bar height, tab bar in various places, eg: if we have translucent header then to set insets on underlying ScrollView.
• In landscape users may decide to show the status bar on iOS (even though this isn't the default behavior), we should adjust the header accordingly

The current SafeAreaView implementation handles some of this but it has limitations (async layout measurement and positioning for example).

Let's use this issue to collect problems that we need to solve related to this.