[DO NOT MERGE]Move offload start related flags from BTA_AV_SCB to

BTA_AV_CB

Ignore a2dp offload start request when offloading already in progress.

Tag: #stability
Bug: 203127797
Test: Force to start offload duplicated
Change-Id: I99858852866e71d60200a798dd31d61a20b2669c
Merged-In: I99858852866e71d60200a798dd31d61a20b2669c