Add String16::c_str and start using it.

This is for parity with String8::c_str and in general the first step
in transition from String8|16.string() - to make it more similar to
std::string.

Bug: 295394788
Test: mma -j
Change-Id: I6c1411bef07c761fb2fb3fb38c27801ac4cffc57
4 files changed