fix: support custom fontFamily configurations in headerTitleStyle

[Kasendwa]

Passing a custom font for the headerTitleStyle configuration is ignored because the user set values always get overridden by the fonts values from the useTheme() hook.

By changing the order of the values passed to Stylesheet.flatten(), any user passed fontFamily configurations then get retained.

I believe this fixes the issue raised in #11429. It's not just iOS specific; it cuts across all platforms.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 77.14%. Comparing base (3799b16) to head (5104ced).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #11880 +/- ## ======================================= Coverage 77.14% 77.14% ======================================= Files 210 210 Lines 6270 6270 Branches 2474 2474 ======================================= Hits 4837 4837 Misses 1379 1379 Partials 54 54 ```

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