libutils: add String8 and String16 std_string utility methods

Add utility methods that convert String16 and String8 into
std::string.

Also, remove a repeated include of <utils/Unicode.h> in
String16.h, since it is not used in that header file,
and is already included in String16.cpp.

BUG: 27200800
Change-Id: I5238aeb70689499763060a99dff9950fbb7adb3e
TEST: libutils builds successfully.