Migrate threading model to Coroutines - long running branch

[wzieba]

Description

This PR groups PRs migrated the SDK from AsyncTask/java.utils.Timer APIs to Kotlin Coroutines. The migration has a few goals in mind

• stop using deprecated APIs
• cover SDK with time-agnostic unit tests
• increase stability and robustness by various fixes

Contains changes from:

• https://github.com/Parsely/parsely-android/pull/86
• https://github.com/Parsely/parsely-android/pull/91/
• https://github.com/Parsely/parsely-android/pull/92/
• https://github.com/Parsely/parsely-android/pull/93/

[codecov[bot]]

Codecov Report

Attention: 24 lines in your changes are missing coverage. Please review.

Comparison is base (1e6e189) 51.22% compared to head (ec44c99) 68.24%.

Files	Patch %	Lines
...ava/com/parsely/parselyandroid/ParselyTracker.java	60.00%	7 Missing and 1 partial :warning:
...va/com/parsely/parselyandroid/EngagementManager.kt	82.85%	3 Missing and 3 partials :warning:
.../parsely/parselyandroid/AdvertisementIdProvider.kt	84.21%	3 Missing :warning:
.../java/com/parsely/parselyandroid/JsonSerializer.kt	75.00%	2 Missing and 1 partial :warning:
...in/java/com/parsely/parselyandroid/FlushManager.kt	81.81%	1 Missing and 1 partial :warning:
.../java/com/parsely/parselyandroid/InMemoryBuffer.kt	95.65%	0 Missing and 1 partial :warning:
...com/parsely/parselyandroid/ParselyAPIConnection.kt	90.90%	0 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #89 +/- ## =========================================== + Coverage 51.22% 68.24% +17.01% =========================================== Files 10 16 +6 Lines 367 359 -8 Branches 56 52 -4 =========================================== + Hits 188 245 +57 + Misses 165 97 -68 - Partials 14 17 +3 ```

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