Clean up and expand long screenshots test coverage

This combines two nearly duplicate sets of tests into a single reusable
base test class and adds test coverage for ListViewCaptureHelper and
WebViewCaptureHelperTest.

The tests cover scrolling, bounds checking and clipping and ensure
the captured region is centered vertically within the visible bounds.

Bug: 195109744
Test: atest ListViewCaptureHelperTest RecyclerViewCaptureHelperTest \
      ScrollViewCaptureHelperTest WebViewCaptureHelperTest
Change-Id: Ia533d38238d2a0e0f44358d2033394ad1d70487f