Support heap dumps generated with HotSpotDiagnosticMXBean

Changes required:
* Add support for heap dumps with id size 8
* Be tolerant of the use of undefined string id 0
* Normalize class names such as "[Ljava/lang/Class" to "java.lang.Class[]"

Also:
* Add a python script to dump a binary heap dump to text, which has been
  historically useful for diagnosing issues like this one.

Bug: 111147686
Test: m ahat-tests, with new test added.

Change-Id: I3cb931fbf7614986b3381f57dcc8069f68740cf1