Split up FlowDumper implementations.

* FlowDumperImpl is not exclusively for subclassing.
* ActivatableFlowDumperImpl self registers whenever it is activated.
* SimpleFlowDumper has to be manually dumped.

Flag: EXEMPT unflaggable refactor
Test: atest SystemUITests
Bug: 357987442
Change-Id: I5501b4878e7fe30f6a6d19b08b327ace009ad9c7