setImageResource and ShapeDrawable Issue

When updateDrawable(null) is called, the default drawable is set as mContentDrawable. Later when we attempt to set the image we try to confirm that there is no content drawable before proceeding which fails because the default is now the content. To correct this, I made an addition to check that mContentDrawable is not mDefaultDrawable.

The second issue is that on Android 4.2, ShapeDrawables must not use the default constructor because if mutate() is called, a NullPointerException is thrown.