Skins are going through measurement even when their component has explicit size and no measurement is needed

Steps to reproduce:
1. Run attached files
2. Notice trace output indicating that skin is going through measurement.
3. Now remove the explicit width/height from the button
4. run
5. notice trace output indicating that the skin is going through measurement twice.

Expected Results:

We have explicit optimizations in the framework to skip measurement phase when we know it is unnecessary. FxComponent skinning model breaks this optimization, because skins never have explicit sizes assigned to them, but no measurement should be necessary if the component has an explicit size (or if the compoennt decides measurement is unnecessary for other reasons).

Furthermore, the skin shouldn't have to go through measurement twice, given that it's only being displayed once. We need to make sure we're doing the minimal amount of work on startup, and multiple measurement passes aren't helping.

Workaround (if any):

None.