Demo: show scroll chip and capture tall screenshot when tapped

The result is shown via intent ACTION_VIEW, after saving to PNG.

This change introduces ScrollCaptureClient, a light layer around
the raw binder API for scroll capture (which is more disruptive
to change). This provides a layer to help adapt patterns between
the app interface and system UI, as well as provide an optimal
testing strategy.

Test: atest ScrollCaptureClientTest
Change-Id: Ib6d0f61ae0b4d4dc3df459e07d9eaf34d11d59ed