audio encoders: fix initial timestamp

Use timestamp from the first non-empty work, not the very first one
which may be empty with an invalid timestamp.

Bug: 123428627
Test: atest CtsMediaTestCases:MediaRecorderTest
Change-Id: I50f68765355b1fd3af4241adad0c6199fd7b4de8
diff --git a/media/codec2/components/aac/C2SoftAacEnc.h b/media/codec2/components/aac/C2SoftAacEnc.h
index 82fb438..779365b 100644
--- a/media/codec2/components/aac/C2SoftAacEnc.h
+++ b/media/codec2/components/aac/C2SoftAacEnc.h
@@ -57,6 +57,7 @@
     UINT mOutBufferSize;
 
     bool mSentCodecSpecificData;
+    bool mInputTimeSet;
     size_t mInputSize;
     c2_cntr64_t mInputTimeUs;