When deregister bta_sys_deregister set nullptr am: 5922eec9

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Bluetooth/+/2056574



Change-Id: Ic5b152f7e92d634e07ab3e13a6ae80c351f20173
Signed-off-by: Automerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>