Changes

amr_trau: add osmo_amrt_fill_with_dhf() function

It is useful to have a function that fills struct osmo_amrt_if
with constant DHF bits.  In an E1 Abis MGW this function is needed
in order to emit DHFs on Abis DL during initial or prolonged absence
of RTP input; it can also be used in a transcoding CN MGW to emit
DHFs in RTP on AoIP interface during similar conditions of no G.711
input being transcoded or TFO-handled.

Change-Id: Ie085e593d6326072295a8cedf2c2c103883f93d5