Changes

AMR codec: add decoder homing frame const patterns

ETSI-defined decoder homing frames for HRv1 and EFR are already
captured in osmo_gsm620_homing_frame[] and osmo_gsm660_homing_frame[];
now add 3GPP-defined DHFs for AMR, all 8 modes.

These AMR DHFs are useful to have in libosmocodec:

* Future AMR-capable tw-e1abis-mgw will emit DHFs toward the BTS/CCU
  during prolonged absence of RTP input - needed because the alternative
  of emitting an endless stream of No_Data frames may produce undesirable
  effects on Um with some E1 BTS models.  If it is desired to bring
  tw-e1abis-mgw functionality into mainline Osmocom at some point,
  DHF bits need to reside somewhere in libosmo*.

* In future improvements of AMR support in osmo-bts-trx, it may be
  likewise desirable to transmit DHFs on Um downlink during the same
  conditions of prolonged absence of RTP input.

Change-Id: I1f8f8ec36cc13270a1d1fa633583107317abe894