Use debug signing configs for local release build & missing proguard rules

[ambushwork]

What does this PR do?

• Use debug signing configs for local release build of sample app
• Add missing proguard rules for sample app

Motivation

For the convenience of local release build test

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.37%. Comparing base (caa2a5d) to head (9e47213).
Report is 4 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #2344      +/-   ##
===========================================
- Coverage    70.53%   70.37%   -0.16%     
===========================================
  Files          740      740              
  Lines        27630    27630              
  Branches      4627     4627              
===========================================
- Hits         19487    19442      -45     
- Misses        6878     6896      +18     
- Partials      1265     1292      +27     

see 23 files with indirect coverage changes