Bluetooth: AVRCP: Implemation of AVRCP.

This patch implementing avrcp.c
New Kconfig BT_AVRCP is provided to enable this layer.
BT_AVRCP_TARGET and BT_AVRCP_CONTROLLER are then
provided to enable one of the two roles independently.
avrcp.h shows the APIs for the upper layer.

Only connection and disconnection interfaces are provided in this patch.

Signed-off-by: Zihao Gao <gaozihao@xiaomi.com>