[PLAT-9307] add autorelease pool to fix memory leak

[kstenerud]

Goal

BSGAppHangDetector was running a pthread but was calling objc methods with no autorelease pool, causing a memory leak each time it detected an app stall.

Changeset

Wrap the thread's main function in an @autoreleasepool

Testing

Verified the leak and fix with the allocations profiler on a real device.

[github-actions]

Bugsnag.framework binary size increased by 176 bytes from 706,120 to 706,296

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +1.7%     +72  +1.7%     +72    Lazy Binding Info
  +0.0%     +56  +0.0%     +56    String Table
  +0.0%     +52  +0.0%     +52    __TEXT,__text
  +0.0%     +32  +0.0%     +32    Symbol Table
  +1.1%     +24  +1.1%     +24    __TEXT,__stub_helper
  +1.2%     +24  +1.2%     +24    __TEXT,__stubs
  +1.0%     +16  +1.0%     +16    Indirect Symbol Table
  +1.2%     +16  +1.2%     +16    __DATA,__la_symbol_ptr
  -0.2%     -16  -0.1%     -16    [__DATA]
  -0.4%    -100  -0.4%    -100    [__TEXT]
  [ = ]       0  -1.2%    -176    [__LINKEDIT]
  +0.0%    +176  [ = ]       0    TOTAL

Generated by 🚫 Danger