Implicit const char* cast removed, operator<< added to 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 for marlin, bullhead, bat
Change-Id: I6ba76522ef65aa211bc156144990ad8b7495b051
4 files changed