Add a struct wrapper for bytes fields in stats_log cpp APIs.

To avoid conflict between statslog.write() function signatures for

Atom1 {
  Foo foo = 1 [logMode=bytes];
}

and

Atom2 {
  string bar = 1;
  int64 arg2 = 2;
}

Bug: 122571213
Test: manually tested with new atoms.
Change-Id: Ied0f0bd81cef8d0964f571e921f47022301157d9