Fixing errors in position information of scrollable views reported for accessibility.

1. ScrollView/HorizontalScroll view were reporting only the scroll X and Y but
   failed to convey the max scroll along X and Y so the position can be determined.

2. WebView was not reporting correctly its scroll position for accessibility.

3. Some descendants of AdapterView were reporting incorrect position information.

4. Updated the accessibility docs with some details about the scroll information.

5. Cleaned up duplicated code.

bug:5412132
bug:5412265

Change-Id: I165e73ecde027dad811425b9f395a3f758c923ba