Changes callback to a WeakReference in ActivityContainerCallback.

This change prevents the Runnable posted by ActivityContainerCallback
from retaining the ActivityView's callback if it is never cleared out
from ViewRootImpl.sRunQueues.

Bug: 19872883
Bug: 19654978
Change-Id: I6dce4381b96c8c77afcd38a55bfe474f13dfbfba
1 file changed