Fix a missing synchronization to Notification

Concurrent access on allPendingIntents could cause
crash by ArrayIndexOutOfBoundsException.
Below two accesses can occur in two different threads.
But allPendingIntents(ArraySet) is not thread-safe.
1. Access to write.
   allPendingIntents = new ArraySet<>();
   parcel.writeArraySet(allPendingIntents)
2. Access to add.
   allPendingIntents.add(intent);

So, we added missing synchronization to Notification

Test:
It is hard to reproduce by manual.
Make/Update notification with pendingintent.

Change-Id: Ib866f6b92528f7a944ac93997a9cff07892d4192
Bugs: 144081764