Changes

E1: replace idle_tf_fr[] with a better version

The only truly correct way to pass FR/HR/EFR traffic from an
incoming RTP stream to TRAU-DL frames is to apply the transform
of TS 28.062 section C.3.2.1.1, originally specified for TFO
but also necessary in the non-3GPP-specified case of GSM TrFO
as it happens here.  Unlike the situation with EFR, a FOSS
implementation of TFO transform for FRv1 does exist in Themyscira
libgsmfr2 - however, making these Themyscira codec libraries
usable from mainline Osmocom programs by way of a proposed
libosmocore DSO plugin mechanism, followed by a major redesign
of OsmoMGW-E1 to pass all DL traffic through this TFO transform,
would be a major project, difficult to justify prior to development
of a proper TFO transform for EFR to complement the one for FRv1.
Hence no change is being made currently to the arguably flawed
general architecture of OsmoMGW-E1, which consists of passing
through unaltered all valid codec frames that are received in RTP,
and inserting fixed dummy TRAU-DL frames when no RTP-derived ones
are available.

The original dummy TRAU-DL frame for FRv1 exhibited the following
defects:

* The payload frame transmitted to the MS consisted of all zero
  bits.  Standard type-approved GSM MS would interpret this bit
  pattern as a SID frame, even though the BTS is told via C16 bit
  "this frame is not a SID".  Passing a SID is not inherently bad
  in itself in this context, but a SID with all LARc parameters
  set to 0 is a strange/poor choice.

* The setting of C-bits (defined in TS 48.060 section 5.5.1.1.1)
  was a mish-mash between TRAU-UL and TRAU-DL frame formats,
  thereby not matching the standard TRAU-DL C-bits fill produced
  by osmo_rtp2trau().

The replacement fill frame introduced here differs as follows:

* The payload bit content is the silence frame of GSM 46.011
  Table 1: certainly better than a SID with all LARc parameters
  set to 0, and arguably better than any SID at all.

* The full TRAU-DL frame (C-bits pattern, peculiar ordering of
  D-bits) was generated by passing said silence frame through
  osmo_rtp2trau(), using this ad hoc tool:

https://gitea.osmocom.org/themwi/dummy-dl-frames

Change-Id: I995824586058e4e4ed77e900d4b57e5113f9eff6