Fix Class::IsInSamePackage to not read beyond the end of a StringPiece.

Fix length of string piece to be size_type rather than int because expecting
negatively sized strings is a bad idea and we should use unsigned types to
defensively guard against mistakes.
Remove max_size and capacity since the return type is inconsistent between
Google and the STL, and we don't need the functions.
Add a bound check in libartd to operator[] accesses.

Change-Id: I1b87a03d8fbd95e7dbb106745e304d1083898075