[system][bt] fix -Wdangling-gsl

BtAddrString() returns a std::string. It's not safe to chain a call to
c_str() as otherwise the returned std::string is a temporary, and the
expression evaluates to an immediately dangling pointer.

Bug: 139945549
Bug: 142558228
Test: mm
Change-Id: I30972458abcc563b24ee0d80b289c3efd6c3e04d
Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>