Revert "Revert "Refactor imgdiag in preparation for ArtMethod""

This reverts commit 45de57fe.

Fixes include:
1) Correcting the place where zygote_diff_pid is moved to image_diff_pid
for the zygote only use-case.
2) Fixing nullptr accesses due to incorrect use of class_dirty, etc.

Original commit message:

Pull the memory mapping setups and the generic byte-diffing code out of
the primary dump method.  This makes it easier to see the mirror::Object
portions of the code that need to be replicated now that ArtMethod is
no longer in the heap.

Bug: 62554875
Test: make -j 40 test-art-target-gtest-imgdiag_test

Change-Id: I59dfae513cc95121f38cdcb3bff3de2557db379d