Ensure root render node clip cannot expand beyond dirty area

Use new Skia API to enfore clip restriction for root render node.
This brings Skia pipeline in line with HWUI. Unit test is
updated to reflect the new behaviour.

Test: Built and ran angler-eng, ran HWUI unit tests.

Change-Id: Iffce70fd37b6aff45eb6a23c8b1a64f45b5f1463