Fix a memory leak when disabling AVDTP

Added new function bta_av_free_scb() to explicitly
free tBTA_AV_SCB struct.

Bug: 78034873
Test: Code compilation
Change-Id: Ic738c64f61fd273bc2988fa98cb7cedebf1c4b76
Merged-In: Ic738c64f61fd273bc2988fa98cb7cedebf1c4b76
(cherry picked from commit b4bd9ee17e242be9a8707246e2e9346e66bc8b1f)