Increase javax.obex performance

When a file is transferred over OBEX the javax.obex classes allocate
the same amount of memory multiple times simply to copy the same data
around internally, only to throw those allocations away when the next
data block is processed. In the Bluetooth OPP case that meant that
64Kb was allocated around 15 times for every 64Kb transferred. Since
the transfer speed is around 1.3Mbps that means almost 2.5Mb of
memory is garbage collected every second, increasing the time needed
for each block transfer.
Fixed by keeping data in reusable byte buffer objects instead.
The time to transfer a 39MB file is reduced by about 10-20%.

Change-Id: I182c0374c2915d1b37ca12200fb36de57dabc67e