messageTray: Add source policy setter
Commit 8f15193b changed the policy
property from a regular JS property to
a getter. This was necessary to avoid calling an overridden _createPolicy()
method before a subclass is properly initialized, but it broke the second
way of using notification sources:
Don't create a Source subclass, but use the base class directly and change
its policy
property.
There's no good reason why we should no longer allow this, so add a setter.