layers: Reorganize ObjectTracker for future codegen

Moved to-be-generated source to object_tracker.cpp, manually-
written and helper functions to object_tracker_utils.cpp (new
file), and reworked header. Updated doc validator to search
new file.

Change-Id: I0fd3bcc4f6d123db07c7f8e2633e6fe16833e231
diff --git a/scripts/vk_validation_stats.py b/scripts/vk_validation_stats.py
index 3c930f3..2e7922d 100755
--- a/scripts/vk_validation_stats.py
+++ b/scripts/vk_validation_stats.py
@@ -57,6 +57,7 @@
 '../layers/descriptor_sets.cpp',
 '../layers/parameter_validation.cpp',
 '../layers/object_tracker.cpp',
+'../layers/object_tracker_utils.cpp',
 '../layers/shader_validation.cpp',
 '../layers/buffer_validation.cpp',
 ]