Make ImageWriter and PatchOat helper classes nested.

Nested classes are better for encapsulation. For example,
we've had an art::FixupRootVisitor in both image_writer.cc
and patchoat.cc and we didn't get an ODR violation only
because these two files are never used in the same process.

Test: Rely on the TreeHugger.
Change-Id: I39a86994923a9c0faa5f05d8c9ede982ce9c0c4c