Launch ahat server before processing the heap dump.

Change-Id: Ic8f18ad3010cb0937f5ef68228359258ec4793fd
Test: Manually launch ahat on a large heap dump with ahat already
      running. Verify that ahat fails immediately.
Test: Manually launch ahat on a large heap dump without ahat already
      running. Verify that visiting localhost:7100 immediately
      eventually resolves to the overview page rather than say the
      site can't be reached.
Bug: 28611294