fix: Allow passthrough of safeAreaInsets on Appbar.Header

[chrysb]

Motivation

If you use Appbar.Header within a <SafeAreaView> — there's no way to remove the top padding if you don't want it.

safeAreaInsets is a valid prop for this component, but setting it does nothing.

    <Appbar.Header
      safeAreaInsets={{ top: 0 }}
    >

This PR allows developers to have control over the insets of this component.

Test plan

This is a two line change, but as I'm not intimately familiar with this repo, I might be overlooking something.

To test, simply modify the safeAreaInsets on the Appbar.Header to modify the top padding.

[callstack-bot]

Hey @chrysb, thank you for your pull request 🤗. The documentation from this branch can be viewed here.