dumpstate: Further cleanups related to new stack dumping scheme.

- Rewrite AddDumpFds to iterate over all files in the dump directory
  instead of assuming a specific file pattern. This gives us additional
  flexibility in generating dump file names.

- Fix a bug in deducing the value of is_global_stack_trace_file.

- Add a better section header for the most recent ANR dump. The most
  recent dump will be titled "VM TRACES AT LAST ANR" and all others will
  be titled "HISTORICAL ANR". Also fixes the sort order so that the most
  recent ANR shows up first.

Bug: 36024548
Test: Manual; adb bugreport

Change-Id: I461712954d033ef3a3911d5b419a101cac71c8fb