[PLAT-8292] Fix duplication of app and device data in session payloads

[nickdowell]

Goal

Stop app and device from being included inside the session JSON object in addition to being in the top-level payload sent to sessions.bugsnag.com.

Changeset

Constructs the payload using a dictionary literal instead of -[BugsnagSession toJson] (which builds a payload suitable for when a session needs to be saved to disk.)

Prior to #1246 -[BugsnagSession toDictionary] was used - but that is also unsuitable for use here since it adds the unwanted "handledCount" and "unhandledCount" fields.

BugsnagSession has multiple serialization methods and it's not obvious where each should be used. A follow-up PR will refactor this to reduce the chance of similar bugs being introduced in the future.

Testing

Altered the E2E steps to very session payload keys for event and session payloads.

[github-actions]

Bugsnag.framework binary size did not change - 857,752 bytes

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.1%    +376  +0.1%    +376    __TEXT,__text
  -2.4%    -376  -2.4%    -376    [__TEXT]
  [ = ]       0  [ = ]       0    TOTAL

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