SensorService performance improvements.

  i) Send ack for wake_up sensors on the socket connection instead of using Binder RPC.
  ii) Cache events per connection in case there are write failures. Compute cache size
      from FIFO counts of sensors.
 iii) Send FlushCompleteEvent only for apps that explicitly called flush().

Change-Id: I018969736b7794b1b930529586f2294a03ee8667