common: fix a potential memory leak after clear()

common: fix a potential memory leak after clear()

Paints must clear canvas engine data if they were dismissed from the canvas,

1. Canvas::clear(free = false) must retain all the paints from the paints hierarchy
so that user keeps the all dangled paints lifecycle.
In this scenario, it could leak the engine data of paints, this patch fixes it.

2. Previously, t just keeps the immediate paints lives of canvas, but not them of children of scene nor picture.
This patch changes a policy which was not considered seriously,
Now it keeps the all paints lives through the tree-hieararchy.

3. Also changed the Scene::clear() behavior identical to Canvas::clear() for consistency.

@API Modification:

Change-Id: I58708f7c759f48be30853364da87b70d63adbe22
From: Result Scene::clear() noexcept;
To: Result Scene::clear(bool free = true) noexcept;