[opus] Reset nb_no_activity_frames when analysis DTX isn't used

Cherry-picked from
https://github.com/xiph/opus/commit/cdaf661e8d3e85770bf06db8cff12ae6be7fa2a6

The patch is needed for OPUS_GET_IN_DTX to work in Celt-only mode.

Bug: b/141607706
Change-Id: Ifbf2cc13d2a03f410aeff073550b185e0d535f91
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1825520
Auto-Submit: Gustaf Ullberg <gustaf@chromium.org>
Commit-Queue: Felicia Lim <flim@chromium.org>
Reviewed-by: Felicia Lim <flim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700251}

third_party/opus/README.chromium

@@ -23,3 +23,4 @@ Local changes:
 * Apply https://github.com/xiph/opus/commit/4f4b11c2398e96134dc62ee794bfe33ecd6e9bd2
 * Apply https://git.xiph.org/?p=opus.git;a=commit;h=812ae3fb5c589aaafe761b8ebf86bcbbb8f0ed76
 * Apply https://git.xiph.org/?p=opus.git;a=commit;h=cd529ed
+* Apply https://github.com/xiph/opus/commit/cdaf661e8d3e85770bf06db8cff12ae6be7fa2a6

third_party/opus/src/src/opus_encoder.c

@@ -2142,6 +2142,8 @@ opus_int32 opus_encode_native(OpusEncoder *st, const opus_val16 *pcm, int frame_
           RESTORE_STACK;
           return 1;
        }
+    } else {
+       st->nb_no_activity_frames = 0;
     }
 #endif