btif_rc: Don't memset STL mutex objects

Switching to STL mutex/recursive_mutex objects left us with
code which memset these objects.  These are opaque, and need to
be initialized/destroyed via constructors/destructors, not
through directly setting their memory.

We change our memset calls to only zero out the parts of the
structs which don't have these objects.

Test: Recompile
Change-Id: Idf666e935f4094ad4cfca7cdafa523f777a7e769