Fix use-after-free while setting advertisement data.

Producer context was freeing data buffers without
performing a deep copy on the passed structure.

Also added some const correctness so we can reason
about the state of btif_adv_data_t as its data
gets associated with the advertising instance.

Bug: 21343750

Change-Id: Ifea0209e8426b61d8456c299c836ba640bd30f31
3 files changed