Fix issue with loading jar resources on Windows.

Hopefully fixes #1049.

The issue is that ```ShadowAssetManager.openNonAsset()``` receives a file name prefixed with "jar:", which is then passed to ```ResName.qualifyFromFilePath()```, that creates a new ```FileFsFile```. The constructor runs ```file.getCanonicalFile()```, which throws an IOException:


```
Caused by: android.content.res.Resources$NotFoundException: File jar:C:\Users\someone\.m2\repository\org\robolectric\android-all\4.3_r2-robolectric-0\android-all-4.3_r2-robolectric-0.jar!/res/drawable-hdpi/ab_solid_dark_holo.9.png from drawable resource ID #0x1140007
...
Caused by: java.lang.RuntimeException: java.io.IOException: The filename, directory name, or volume label syntax is incorrect
	at org.robolectric.res.FileFsFile.<init>(FileFsFile.java:23)
	at org.robolectric.res.ResName.qualifyFromFilePath(ResName.java:77)
	at org.robolectric.shadows.ShadowAssetManager.openNonAsset(ShadowAssetManager.java:163)
	at android.content.res.AssetManager.openNonAsset(AssetManager.java)
	at android.content.res.Resources.loadDrawable(Resources.java:2102)
```

This does not happen on Linux or Mac. Only Windows.
Removing the "jar:" prefix seems to work, as the file path is then valid on Windows as well.

I do not know how to create a unit test that fails on the build server, which I assume is running Linux.
But the existing tests fail on Windows.

I have not verified this exact fix on Windows yet. 
Can test tomorrow.