Sensor Batching Bug fixes.

i) SensorService dropping events. Increase SOCKET_BUFFER_SIZE in BitTube ctor.
ii) Call flush before every activate.
iii) Emulate flush for older devices. Add a trivial flush complete event when flush is called.

Bug: 10641596
Change-Id: I30d0f3948e830457143f16e157b6ad81908687ce