[qemu]: Move qemu_pipe.h from include/system/ directory.

The <hardware/qemu_pipe.h> header isn't related to any loadable
hardware module, this patch thus relocates it to <system/qemu_pipe.h>
which is a much more logical location.

Note the following changes:

  - The pipe name must begin with the 'pipe:' prefix to avoid an
    un-necessary snprintf() and buffer copy.

  - Does not probe for the obsolete /dev/goldfish_pipe device, i.e.
    only use /dev/qemu_pipe instead.

  - Use QEMU_PIPE_DEBUG() instead of D() as the debugging macro.

+ Update ADB to use the newest <system/qemu_pipe.h>

+ Add qemu_pipe_frame_send() and qemu_pipe_frame_recv() utility
  functions which replace the obsolete qemud_channel_send() and
  qemud_channel_recv() from the defunct <hardware/qemud.h>
  header.

BUG=25875346

Change-Id: Ic290a5b79d466c2af64b49bd9134643277c11bfd