Refactor build_release and test code

Minor restructuring of the build_release pruning code to make it easier
to add logic for handling fields of maps of structs. That includes:
1. Moving some code that is specific to clearing a selected field
   inside the associated if block.
2. Replacing an if with a switch.
3. Improving the error handling by separating the reporting of the
   container that broke from information about which field could not be
   set. That allows follow up code to provide information about the map
   key instead.

The tests were restructed by:
1. Switching from using AssertDeepEquals to compare the structs to
   comparing the output of marshalling the structs to JSON. That was
   for a couple of reasons. Firstly, because JSON will marshal (and so
   allow comparison of) the contents of pointers to structs whereas
   AssertDeepEquals will just compare the pointers themselves.
   Secondly, because JSON can pretty print the output and make it
   easier to read.
2. Using a func to create a new instance of the input structure for
   each test. That is to allow the test to modify the input structure,
   e.g. by clearing a field in a struct that is pointed to by a map.
   The test previously relied on the input structure being immutable
   and passed by value but a follow up change will change that by
   adding a map field that contains pointers to structs.

Bug: 204763318
Test: m nothing
Change-Id: I84dc99621497b7263e30466895b823eb02cb2b56