Fully synchronize access to gatt connections

java.util.ConcurrentModificationException could be thrown when accessing
mConnections using an iterator. To prevent this, fully guard synchronize
access to it and also make it private to avoid unsynchronized external
access.

Tag: #stability
Bug: 178334266
Test: manual, build ADT3 on internal repo and connected remote
Change-Id: Idb14c7e5aeab076db765aca88b1067148f35ec81