Long screenshots framework update and API unhide

This change includes a series of API review changes. The most
significant update is the addition of cancellation signals to
API methods where needed.

Renames some internal classes and interfaces to support the
API changes and to simplify tests and improve quality.

Test: numerous presubmits
Bug: 175830670
CTS-Coverage-Bug: 180419562
Change-Id: I9b1b950a2779fc902ecf4d6394e3c35171926700