Implement takeScreenshot and add driver test for Fuchsia

[blasten]

• Implements flutter screenshot targeting a Fuchsia device.
• Driver test that takes a screenshot and diffs it against the golden one. (The caveat is that the test needs to run on a NUC so the screen size matches. Later, we can expand that support.)

cc @iskakaushik

[zanderso]

Not super familiar with this flow, but it looks like you can set the file output name in the flutter screenshot command, but ResidentRunner.screenshot() hardcodes 'flutter.png'.

[blasten]

If it sounds reasonable, I can follow up with adding a defaultScreenshotName to Device and then let ResidentRunner.screenshot() read the value.

[zanderso]

sg. it looks like the resident runner screenshot call is already using getUniqueFile, so Device might need both a defaultScreenshotName and a defaultScreenshotExt.

[jonahwilliams]

We shouldn't be committing any new binary files into the git repo. @Piinks might know if there is a way we could use skia gold here, or else it might require a secondary repo somewhere

[Piinks]

@blasten and I talked offline. I believe there should be away to just plug in to the Gold dashboard. We might be able to use the flutter_goldens_client package directly, or go through the flutter_goldens package.. this hasn't been done before, and I would love to see if we can continue to extend the Gold platform.

[blasten]

PTAL

[zanderso]

sg. it looks like the resident runner screenshot call is already using getUniqueFile, so Device might need both a defaultScreenshotName and a defaultScreenshotExt.