setImageResource won't set an image if a default drawable already exists

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.
Android 4.2 Fix - ShapeDrawable must be constructed with Args

In Android 4.2 when using the default constructor, if mutate() is
called, mShape is null and returns a NullPointerException.
Merge pull request #1 from jimmithy/master

setImageResource and ShapeDrawable Issue