Check for negative/zero input in createBitmap()

Adds a check for negative or zero values for Width/Height in
ShadowBitmap.createBitmap(int width, int height, Bitmap.Config config)
implementation. Raises IllegalArgumentException if params are negative
or zero. Follows implementation in android.graphics.bitmap.
Merge pull request #1216 from Graya3000/master

Check for negative/zero input in createBitmap()