Update runhat command to use "am dumpheap".

Instead of sending a kill -10 to write a file to /data/misc, we now use
"am dumpheap" to create a file on /sdcard.

Also,
- Removed mention of unused "output-file" argument.
- Quoted args in initial tests so bash doesn't go nuts when you say
  "runhat" with no arguments.

Bug 2835170.

Change-Id: I614fdb54a1d6b8824bc9b568c07cc0901c41a3d4