fix potential use-after-frees of stack memory

`devname_value` is a local variable; if `property_get` succeeds, we'll
return a pointer to it. Returning a `std::string` instead sidesteps this
problem.

Bug: 190757198
Test: TreeHugger
Change-Id: If9ca733dd21128706f2a9f62e8460b1286631aa5