[NR-169834] Background reporting feature

[ywang-nr]

Ticket: https://new-relic.atlassian.net/browse/NR-169834

What's changed:

1. lift the shutdown process when app goes into the background
2. add feature flag: BackgroundReporting, disabled as default
3. add attribute to capture background reported data using new attribute
4. add supportability metrics to match with both platforms

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 36.84211% with 24 lines in your changes missing coverage. Please review.

Please upload report for BASE (release/7.4.0@7fc8dbf). Learn more about missing BASE report.

:exclamation: Current head 7f0e41b differs from pull request most recent head 4cd6c26

Please upload reports for the commit 4cd6c26 to get more accurate results.

Files	Patch %	Lines
...c/agent/android/agentdata/AgentDataController.java	0.00%	4 Missing and 1 partial :warning:
...wrelic/agent/android/analytics/AnalyticsEvent.java	0.00%	4 Missing and 1 partial :warning:
...n/java/com/newrelic/agent/android/crash/Crash.java	0.00%	4 Missing and 1 partial :warning:
.../com/newrelic/agent/android/harvest/Harvester.java	33.33%	2 Missing and 2 partials :warning:
...m/newrelic/agent/android/harvest/HarvestTimer.java	70.00%	1 Missing and 2 partials :warning:
...a/com/newrelic/agent/android/AndroidAgentImpl.java	60.00%	0 Missing and 2 partials :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## release/7.4.0 #235 +/- ## ================================================ Coverage ? 56.02% Complexity ? 3253 ================================================ Files ? 324 Lines ? 16491 Branches ? 1805 ================================================ Hits ? 9239 Misses ? 6263 Partials ? 989 ```

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