btif: introduce a error struct for btav

Introduce an error struct that contains status, error code, and
optional error message. This struct is intended to be used by all btif_av
interface to provide better status/state updates.

BUG: 240781725
Tag: #floss
Test: ./build.py
Test: emerge-${BOARD} floss
Test: gd/cert/run
BYPASS_LONG_LINES_REASON: Bluetooth likes 120 lines

Change-Id: Ifeb95d18420b933018712d9beadd9f5e4428591c