Implement dumpheap -m.

Adds an option to dumpheap that calls mallocInfo and write the xml produced
to a file.

Bug: 36871013

Test: Run am dumpheap -m <FILE>, am dumpheap -n <FILE>, am dumpheap <FILE>
Test: and verified that they produce the correct data.
Change-Id: Ifb9e878ea621c7cacc3aa6e3a889c414681dee2d