Limit data accepted by GAP_ConnWriteData to just one SDU

This limit is already enforced in BluetoothSocket.java, which is
dividing data into pieces no bigger than MTU.

This is first step towards making it accept BT_HDR with data, in order
to make LE sockets work with zero copies.

Test: transfer file using OPP
Bug: 68359837
Change-Id: I67bfd4ab5c2dc6aa8210058a742c57e1ccf845a0