Minor changes to tint color support

[nguyenhuy]

Quick follow up on #1617:

• Need to assert thread affinity in the getter when the node is view-backed and loaded. This is because UIView properties can't be accessed off main thread.
• When the node is view-backed, we shouldn't ascend the node hierarchy. UIView should already ascend the view hierarchy and we currently don't support adding a view-backed node as a subnode of a layer-backed node (the other way around is fine).
• _getFromViewOnly and _setToViewOnly check if the node loaded and do the right thing. So by checking layerBacked first, we can use them and simplify our getter and setter.