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.