Fix crash while turning bluetooth on, due to a race condition.

Bug: 2561150

Race condition: We were basing whether the event loop is
running or not on whether nat->pollData was null or not.
In rare cases, eventLoopMain would have not started,
isEventLoopRunning returns true; java side makes a dbus call
and the watches get set to null, causing a crash in
dbus_connection_set_watch_functions when eventLoopMain starts.

Change-Id: I863f182185a8e956fd53cb58783b7fe0ecfb2ddb