Fix buffer overflow in btif_dm_data_copy

When we use a union, we should always define variables as the union type,
not as one of the field subtypes. If the latter is cast to the union type,
buffer overflow can happen.

Bug: 110166268
Test: compilation
Change-Id: I473c03b099ad5a326e7a3739f65efd33cf4775bd
Merged-In: I473c03b099ad5a326e7a3739f65efd33cf4775bd