Use std::shared_ptr for SpriteController

Remove RefBase from SpriteController, and use std::shared_ptr. We cannot
migrate to std::unique_ptr because we have to post messages to the
handler, which needs to have a weak reference to the object.

Bug: 278783893
Test: presubmit
Change-Id: I0ea4bb220e5b1866375ed39335f9035cd4bb766c