Add interface for payload message collator

[fractalwrench]

Goal

Extracts an interface from the PayloadMessageCollator class. This will allow creation of fakes in tests & also is a precursor refactoring that will enable the creation of the new payload.

Testing

Relied on existing test coverage.

[codecov]

Codecov Report

Attention: Patch coverage is 98.86364% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 80.16%. Comparing base (3b868d3) to head (7d8717f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #551      +/-   ##
==========================================
- Coverage   80.17%   80.16%   -0.01%     
==========================================
  Files         412      412              
  Lines       10986    10986              
  Branches     1617     1617              
==========================================
- Hits         8808     8807       -1     
  Misses       1535     1535              
- Partials      643      644       +1     

Files	Coverage Δ
...race/android/embracesdk/injection/SessionModule.kt	100.00% <100.00%> (ø)
...esdk/session/message/PayloadMessageCollatorImpl.kt	98.85% <98.85%> (ø)

... and 3 files with indirect coverage changes

[lucaslabari]

LGTM