[Android] Handle creating a default GroupHeader if no GroupHeaderTemp…

…late is provided (xamarin#248)

[Pages] Add reference to Xamarin.Forms.Build.Tasks (xamarin#220)

Fix android StartTimer race condition (xamarin#196)

Fix android StartTimer race condition (xamarin#196)

Revert "Remove CarouselPage.xml"

This reverts commit fdcc866.

Remove CarouselView from index.xml

Remove CarouselView from index.xml