DrawableContainer was not respecting the value returned by Drawable.getPadding(Rect).

Before this change, DrawableContainer would always return true from getPadding(Rect)
even if all of its children were returning false from getPadding(Rect). This change
modifies this behavior to respect getPadding(Rect): mConstantPadding is kept null
when getPadding(Rect) returns false for all of the children and a flag is set
to avoid recomputing that value every time getConstantPadding() is invoked.