fix(Scope): $broadcast and $emit should set event.currentScope to null

When a event is finished propagating through Scope hierarchy the event's `currentScope` property
should be reset to `null` to avoid accidental use of this property in asynchronous event handlers.

In the previous code, the event's property would contain a reference to the last Scope instance that
was visited during the traversal, which is unlikely what the code trying to grab scope reference expects.

BREAKING CHANGE: $broadcast and $emit will now reset the `currentScope` property of the event to
null once the event finished propagating. If any code depends on asynchronously accessing thei
`currentScope` property, it should be migrated to use `targetScope` instead. All of these cases
should be considered programming bugs.

Closes #7445