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.
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