LP noise threshold mismatch in DTX
In function `dtx_ivas_fx()` in file dtx_fx.c, there are conditions comparing the LP-filtered noise estimation value against a threshold, but the threshold seems not to be used consistently. Namely:
```
LT_16( st_fx->lp_noise_fx, LP_NOISE_LV * 256 )
```
vs.
```
LT_16( st_fx->lp_noise_fx, 3840 /*15 in Q8*/ )
```
where
```
/* _DIFF_FLOAT_FIX_ : lp_noise_fx threshold is different between float (15) and fix (5*256) */
#define LP_NOISE_LV 5 /* LP_NOISE level */
```
Thus to me, the threshold should be reviewed at two points:
- https://forge.3gpp.org/rep/sa4/audio/ivas-basop/-/blob/main/lib_enc/dtx_fx.c#L210 (L235?)
- https://forge.3gpp.org/rep/sa4/audio/ivas-basop/-/blob/main/lib_enc/dtx_fx.c#L397
issue