Move active session down to SessionHandler

[fractalwrench]

Goal

Moves the activeSession property down to SessionHandler & makes it private. This avoids having to pass state between the two classes unnecessarily.

Testing

Updated existing test coverage.

[codecov]

Codecov Report

Merging #94 (1ce5fe5) into master (e724ccd) will decrease coverage by 0.16%.
The diff coverage is 95.45%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
- Coverage   76.00%   75.85%   -0.16%     
==========================================
  Files         314      314              
  Lines       10078    10071       -7     
  Branches     1460     1458       -2     
==========================================
- Hits         7660     7639      -21     
- Misses       1829     1840      +11     
- Partials      589      592       +3     

Files	Coverage Δ
...ndroid/embracesdk/session/EmbraceSessionService.kt	77.58% <100.00%> (-2.42%)	⬇️
...brace/android/embracesdk/session/SessionHandler.kt	81.87% <94.11%> (-6.52%)	⬇️

... and 1 file with indirect coverage changes

[bidetofevil]

LGTM.

One question though: is there a reason (other than inertia) that we are using so many volatile member variables instead of using atomic references? Any time we have mutable state that can be accessed by multiple threads, we should think carefully about synchronization, and while making something atomic doesn't automatically guarantee thread safety, at least it synchronizes access and forces us (in theory) to think about how that is changed.

[fractalwrench]

I think it's mainly inertia & there isn't a good reason why better synchronisation mechanisms aren't being used.