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: None
Test: `adb bugreport` succeeded
Change-Id: I5fe6964266c8c1393da274cb54000521e6ab8a17