bluetooth: avdtp: implement the avdtp fragmentation and reassembly

Define BT_AVDTP_SIGNAL_SDU_MAX Kconfig item to limit the max avdtp signal
SDU size, then use avdtp self defined net buf pool to prepare data. Use
default acl_tx_pool pool do to the fragmentation.

Signed-off-by: Mark Wang <yichang.wang@nxp.com>