Clean up DumpData-related functions

This change cleans a few DumpData-related functions:

* Makes GetDumpFds return the std::vector<DumpData> by value instead of
  by pointer.
* Makes Dumpstate own the list of open tombstone and ANR files.
* Removes two more globals.
* Prevents a potential FD leak.

Bug: http://b/73140330
Test: `adb bugreport` succeeded

(cherry picked from commit 91c2ae50)

Change-Id: I5ceb9e4fc1e123dfd6823a7626492ce0b63cf822