Use new SkAndroidFrameworkUtils method to get linear gradient info.

This allows Skia to remove the SkShaders::asAGradient method.

Test: android.uirendering.cts.testclasses.ForceDarkTests#testLinearGradient

Bug: skbug.com/8941
Change-Id: Id333f1c083c09b65f9caa8639c231e5d7e98b7c4