Bugfixes for on-demand variants

This CL makes the following bugfixes
1. When running mutators on the on-demand variants, use index of the
   last `CreateModule` calling mutator (plus 1) as the starting index,
 rather than 0.
2. Pass the config object when running mutators on the on-demand
   variants.
3. Set relBlueprintsFile so that ctx.ModuleDir returns the same values
   for normal and on-demand variants
4. Set the correct endIndex in the findVariant calls of
   OtherModule*VariantExists

Bug: 448182009
Test: m nothing with ag/36952357 (switch rust transition mutators to
on-demand)

Change-Id: Id7cc0140064dfa4d02bf61a0c7c30f5a542b34ac