[String8] Add string_view ctor

For convenience add a std::string_view ctor to String8.

This allows for easy std::string to String8 conversion.

Test: compiles
Test: atest libutils_binder_test
Bug: none
Flag: EXEMPT trivial
Change-Id: I188362e4045891e1a68ea433a47e08d27490f473
2 files changed