Add ProGuard rules to sample app

[0xnm]

What does this PR do?

Even though minification is not enabled for the sample app, it is still useful to have Proguard rules attached for the cases when we need to test sample app with minification enabled.

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 :white_check_mark:

Project coverage is 83.19%. Comparing base (eb5ca87) to head (d3f69b6). Report is 2 commits behind head on develop.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #2067 +/- ## =========================================== - Coverage 83.26% 83.19% -0.07% =========================================== Files 494 495 +1 Lines 17715 17773 +58 Branches 2681 2685 +4 =========================================== + Hits 14749 14785 +36 - Misses 2238 2256 +18 - Partials 728 732 +4 ``` [see 26 files with indirect coverage changes](https://app.codecov.io/gh/DataDog/dd-sdk-android/pull/2067/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DataDog)