Android: make ScrollContainer use correct widget for viewport

[mhsmith]

Fixes #1604.

The reason this issue appeared in dev36 was because of #1507. Before that, the widget passed to AndroidViewport was only used to get at the Resources object, so all viewports were the same size: a rather unreliable estimate of the size of the whole window. #1507 changed AndroidViewport to return the dimensions of the widget that was passed to it.

So this PR fixes ScrollContainer to create its viewport from the container, not the content.

PR Checklist:

• All new features have been tested
• All new features have been documented
• I have read the CONTRIBUTING.md file
• I will abide by the code of conduct

[mhsmith]

This fixes the case where the ScrollContainer is the top-level widget in the window. But where the ScrollContainer is contained within a Box, there's a separate issue which also affects other platforms (#1624).

[freakboy3742]

Conceptually this makes sense; however, I'm not seeing any behavior change from screenshot provided in #1604:

Is there a behavioral change that I'm not seeing?

[freakboy3742]

/me facepalms.

When you're testing code, it helps to actually use the code you're testing.

Looks good to me.