Extract session ordinal logic to preference service

[fractalwrench]

Goal

Extracts the business logic for incrementing the session number to the preference service. This means SessionHandler no longer needs knowledge about how the ordinal is calculated.

As far as the session number calculation goes, I think the implementation should be thread-safe. Starting a session is always synchronised with a lock, and that's the only place the function is called. Flushing the change to shared preferences is done using apply() which is async, but internally within the OS implementation the change is kept in an in-memory hashmap. The only two scenarios where the ordinal won't be incremented is if I/O fails before the process can terminate (e.g. due to a race), or if storage is wiped by a user (in which case a new device ID/ordinal is created).

Testing

Updated unit test coverage.

[codecov]

Codecov Report

Merging #81 (60f1de2) into master (8cfc0f0) will decrease coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #81      +/-   ##
==========================================
- Coverage   76.03%   76.02%   -0.01%     
==========================================
  Files         313      313              
  Lines       10097    10095       -2     
  Branches     1455     1455              
==========================================
- Hits         7677     7675       -2     
  Misses       1832     1832              
  Partials      588      588              

Files	Coverage Δ
...roid/embracesdk/prefs/EmbracePreferencesService.kt	90.14% <100.00%> (+0.06%)	⬆️
...ace/android/embracesdk/prefs/PreferencesService.kt	0.00% <ø> (ø)
...brace/android/embracesdk/session/SessionHandler.kt	87.71% <100.00%> (-0.22%)	⬇️

[bidetofevil]

LGTM.

Unrelated to this change, we should probably a similar counting for background activities (or account for background activities in this scheme... some how). I suspect we lose more of those than regular sessions given how much of an second-class citizen it seems to be.