[PLAT-7139] Thermal state and Thermal Kill detection

[nickdowell]

Goal

Detect Thermal Kill errors (where the OS terminates an app due to a device overheating), add a thermalState property to the device tab, and add thermal state change breadcrumbs.

Changeset

• Records thermalState (in metaData) and observe thermal state notifications (to add breadcrumbs) on all supported platforms (requires iOS / tvOS 11.0 or later)
• Records critical thermal state in KV-store, and uses this at start-up to classify an unexpected termination as a Thermal Kill instead of an OOM. OOM heuristic has been refactored into BugsnagSystemState as "unexpected termination" detection. Thermal Kill events have the same payload as OOM events (no stacktrace or threads) but different errorClass, message and type.
• Refactored some notification observation code to follow consistent naming pattern and remove duplicates.
• Renamed BSGGlobals to BSGUtils in order to rehome UIDeviceOrientation -> NSString function

Testing

E2E scenario verifies that a Thermal Kill is reported if an app is killed while the thermal state is critical.

E2E scenarios verify that thermal state metadata and breadcrumbs are recorded.

Successful full E2E run: https://buildkite.com/bugsnag/bugsnag-cocoa/builds/3178

Unit test case for OOMs has been amended to verify logic now residing in BugsnagSystemState

[github-actions]

Infer: No issues found 🎉

OCLint: No issues found 🎉

Bugsnag.framework binary size increased by 1,936 bytes from 1,154,472 to 1,156,408

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