This is not a grand refactor yet, it's just cleaning up what we have
already, so that people who keep using this API (e.g. dialogs) have
something coherent to deal with.
The major changes are that Navigator and NavigatorState have the same
API now, that most of the examples use `<void>` instead of `<Null>`,
that the navigator observer can see replaces, and that the `settings`
is moved from ModalRoute to Route. I also cleaned up some of the API
documentation.
* make time picker accessible
* use new CustomPaint a11y API
* flutter_localizations tests; use bigger distance delta
* fix am/pm control; selected values
* fix translations; remove @mustCallSuper in describeSemanticsConfiguration
* exclude AM/PM announcement from iOS as on iOS the label is read back automatically
* alwaysUse24HourFormat in MediaQuery and time picker
* docs; dead code
* address some comments
* MaterialLocalizations.timeOfDayFormat is the single source of 24-hour-formattedness
* Make TimePickerDialog private again
* wire up MediaQueryData.fromWindow to Window
