Consolidating ObjectSubscription and ClassSubscription

Zoom-related tests were previously relying on ObjectSubscription and
ClassSubscription classes as well as AwaitableObserver.
ObjectSubscription was a duplicate of AwaitableObserver. The usage of
ObjectSubscription is replaced by AwaitableObserver and a new class
called ClassAwaitableObserver, which builds on top of AwaitableObserver,
replaces ClassSubscription.

Change-Id: I1375774fe8c8e636fc953e91c420d9eae8d315e0
Signed-off-by: Nicolas Schiper <nicolas@midokura.com>