[Dataflow Streaming] Enabled Heartbeat by Default

[TongruiLi]

This sets heartbeat work by default and will rollout along next beam release. This CL also refactors and fixed some unit test issues.

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 71.37%. Comparing base (5a09095) to head (5c1fa0e).

:exclamation: Current head 5c1fa0e differs from pull request most recent head 3f97733

Please upload reports for the commit 3f97733 to get more accurate results.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #31689 +/- ## ============================================ + Coverage 71.18% 71.37% +0.19% + Complexity 3019 1474 -1545 ============================================ Files 1058 900 -158 Lines 134254 114202 -20052 Branches 3257 1076 -2181 ============================================ - Hits 95571 81515 -14056 + Misses 35536 30659 -4877 + Partials 3147 2028 -1119 ```

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[TongruiLi]

assign set of reviewers

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Reviewers are already assigned to this PR: @robertwb