[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>
(cherry picked from commit 20ed45d6339079645ef9fe576b894e9497684c93)
diff --git a/service/low_energy_client.cc b/service/low_energy_client.cc
index cc127c3..cb8bcf1 100644
--- a/service/low_energy_client.cc
+++ b/service/low_energy_client.cc
@@ -188,8 +188,8 @@
   if (!bda) return;
-  const char* addr = BtAddrString(bda).c_str();
-  if (delegate_) delegate_->OnMtuChanged(this, status, addr, mtu);
+  std::string addr = BtAddrString(bda);
+  if (delegate_) delegate_->OnMtuChanged(this, status, addr.c_str(), mtu);
 // LowEnergyClientFactory implementation