Notify ViewRootImpl when it's safe to modify Canvas.

When Activity.convert{To|From}Translucent() is called the
ViewRootImpl is now notified when it is safe to convert the Canvas
from translucent to opaque and back to translucent. This will make
it possible to save resources when compositing opaque layers.

Fixes bug 10349536.

Change-Id: I7282aee1d54601fb00611d20be204bf164d873f6