Fix sockfd leakage in SensorService.

i) Call removeFd() only if the fd in the BitTube has been
previously added to the Looper. Use a flag to determine whether the fd
has been previously added or not.
ii) Increment mPendingFlushEventsToSend after holding a connectionLock.
iii) Store the number of acks that are pending in SensorEventQueue
 and send them all at once.

Bug: 17472228
Change-Id: I1ec834fea1112a9cfbd9cddd2198438793698502