Fix Android Studio 'Rebuild Project'

[xgouchet]

What does this PR do?

Android Studio's Rebuild Project button performs the equivalent of ./gradlew clean assemble. Because of that the genDir folder used to store json2kotlin generated files is cleared and doesn't exists when the GenerateApiSurfaceTaskis ran.

The recommended way is to store path instead of actual files when the file is not guaranteed to exists when the task is ran.

Motivation

Being able to use the Rebuild Project feature in AS.

[codecov-commenter]

Codecov Report

Merging #1602 (cea3775) into develop (bf9b6c3) will decrease coverage by 0.13%.
Report is 28 commits behind head on develop.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop    #1602      +/-   ##
===========================================
- Coverage    83.58%   83.45%   -0.13%     
===========================================
  Files          443      443              
  Lines        15122    15122              
  Branches      2264     2264              
===========================================
- Hits         12639    12620      -19     
- Misses        1885     1896      +11     
- Partials       598      606       +8     

see 20 files with indirect coverage changes