Add low memory warnings as span events

[leandro-godon]

Goal

Report low memory warnings as span events. The previous collection method has not been removed, so warnings are also sent in the mw key of the PerformanceInfo payload.

Testing

Tested generating a memory warning on a device and observing the span event in the mock api.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.18%. Comparing base (4f3bb3c) to head (8d712de).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #653      +/-   ##
==========================================
+ Coverage   80.16%   80.18%   +0.02%     
==========================================
  Files         417      418       +1     
  Lines       11343    11370      +27     
  Branches     1680     1680              
==========================================
+ Hits         9093     9117      +24     
- Misses       1577     1580       +3     
  Partials      673      673              

Files	Coverage Δ
.../embrace/android/embracesdk/arch/schema/EmbType.kt	85.71% <100.00%> (+1.09%)	⬆️
...brace/android/embracesdk/arch/schema/SchemaType.kt	97.14% <100.00%> (+0.36%)	⬆️
.../embracesdk/capture/memory/EmbraceMemoryService.kt	100.00% <100.00%> (ø)
...bracesdk/capture/memory/MemoryWarningDataSource.kt	100.00% <100.00%> (ø)
...d/embracesdk/injection/DataCaptureServiceModule.kt	100.00% <100.00%> (ø)
...e/android/embracesdk/injection/DataSourceModule.kt	100.00% <100.00%> (ø)

... and 3 files with indirect coverage changes