[Plat-5196] Fix init order

[kstenerud]

Goal

Some parts of the library were being used before initialization.

Design

Make sure the init functions are called before anything uses them. Also, don't allow arbitrary configs to be passed in to the lower levels when there's always a single global instance in reality.

Testing

Reran all tests.

[github-actions]

Bugsnag.framework binary size did not change - 815,960 bytes

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%      +8  +0.0%      +8    [__TEXT]
  +0.2%      +8  +0.2%      +8    __TEXT,__unwind_info
  -0.0%     -16  -0.0%     -16    __TEXT,__text
  [ = ]       0  [ = ]       0    TOTAL

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