fix a few problems with BitTube

BitTube used to send objects one at a time and didn't
handle errors properly.

We now send all the objects in one call, which means they
have to be read as a single batch as well. This changes the
BitTube API.

Update SensorService to the new API.

Also added an API to set the size of the send buffer.

Bug: 10641596
Change-Id: I77c70d35e351fdba0416fae4b7ca3b1d56272251