Memory safe events in bta_dm_disc

This patch explores possibility of replacing malloced memory with
unique_ptr to variant for messages passed in BTA layer.

Such container would be memory safe, and free of any potential leaks.
In such containers, one could pass types like std::vector, and std::list

Bug: 330675788
Flag: exempt, infrastructure refactor that can't be easily isolated
Test: mma -j32
Change-Id: Ia7ea581e8a0d17ea4605d50f303803380b6d6625