btif_a2dp_source: Trace offload a2dp sessions

Metric trace generation was skipped when the a2dp session was
offloaded; resulting in skewed codec statistics.

Bug: 346590002
Test: m com.android.btservices
Test: Manual streaming tests
Flag: EXEMPT, minor bugfix
Change-Id: Ied6372c3c4d751b0fbd10b13372acf181e95affa