Add ClassIterator

Add a way to iterate over all of the classes in a Dex file. The
iterator returns a ClassAccessor for each class.

Added DexFile::GetClasses to return an iteration range for this new
iterator type.

Sample usage:
for (ClassAccessor accessor : dex_file.GetClasses()) {

Bug: 79758018
Bug: 77709234
Test: test-art-host
Change-Id: I66e000aa11f433e72f6857496f4e89a0b811f5a2