Allow "am" to initiate heap dumps.

This was mostly cloned from the "am profile" implementation.  It's
intended to replace the old "kill -10" approach used by "runhat".

We could really use a native heap dump, so I pass a "managed"
flag through that indicates whether we want to dump the native or
managed heap.  We don't currently have a native heap dump-to-file
function, so it currently just logs a warning.

(android.ddm.DdmHandleNativeHeap.getLeakInfo is a good start -- it
copies /proc/maps and then calls get_malloc_leak_info to get some
goodies.  Needs some formatting to make it human-readable.  I didn't
want to cram all that into this change.)

It would be useful if "am" didn't exit until the heap dump operation
completed, but I'm not sure how to do that.

Bug 2759474.

Change-Id: I46bc98067738d8c72ac0fc10002ca67bb4929271