Bluetooth: AVRCP: Add alloc_buf callback and browsing RX pool

Introduce an optional alloc_buf() callback into AVCTP so that each
session can decide which RX buffer pool to use. Provided a dedicated
RX pool for the AVRCP browsing channel.
Also provided BT_AVRCP_BROWSING_L2CAP_MTU for configure.

Signed-off-by: Make Shi <make.shi@nxp.com>