btif_a2dp_source: Call btif_a2dp_source_shutdown_delayed with no scheduling

if already on the same thread. Doing otherwise the caller with block
the main thread waiting on the shutdown completion.

Bug: 374166531
Bug: 384401534
Test: m com.android.btservices
Flag: com.android.bluetooth.flags.a2dp_source_threading_fix
Change-Id: Ib538db147cb800ecb469a7bf104acd3612503e90