Use explicit .c_str() for hidl_string

hidl_string no longer will provide an implicit cast to const char* as it
interfers with other expected behaviors of the class.  It now emulated
std::string in requiring a call to .c_str() to get the same behavior.

Bug:  36532780
Test:  Build the tree
Change-Id: Icfa77d2d505d527f5c9484ea73d2e596ccd810af