audio VTS: Clarify numbers used by CompressedOffloadOutputStreamTest

Make more clear the origin of numbers used by the test
CompressedOffloadOutputStreamTest#Mp3FormatGaplessOffload.

No change in the test behavior.

Bug: 260676244
Test: m VtsHalAudioV7_1TargetTest
Change-Id: I48211ad4510dec7fe90dc6f755485283376eaa23
diff --git a/audio/core/all-versions/vts/functional/7.0/AudioPrimaryHidlHalTest.cpp b/audio/core/all-versions/vts/functional/7.0/AudioPrimaryHidlHalTest.cpp
index 505c54c..55d4e1d 100644
--- a/audio/core/all-versions/vts/functional/7.0/AudioPrimaryHidlHalTest.cpp
+++ b/audio/core/all-versions/vts/functional/7.0/AudioPrimaryHidlHalTest.cpp
@@ -957,10 +957,16 @@
     ASSERT_NO_FATAL_FAILURE(createPatchIfNeeded());
     const int presentationeEndPrecisionMs = 1000;
     const int sampleRate = 44100;
+    // The duration of sine882hz3s.mp3 is: 3 seconds + (576 + 756) samples.
+    // This is a mono file, thus 1 frame = 1 sample for it.
+    const int fullTrackDurationMs = 3000 + (576 + 756) * 1000 / sampleRate;
     const int significantSampleNumber = (presentationeEndPrecisionMs * sampleRate) / 1000;
+    // 'delay' is the amount of frames ignored at the beginning, 'padding' is the amount of frames
+    // ignored at the end of the track. Extra 1000 samples are requested for trimming to reduce the
+    // test running time.
     const int delay = 576 + 1000;
     const int padding = 756 + 1000;
-    const int durationMs = 3000 - 44;
+    const int durationMs = fullTrackDurationMs - (delay + padding) * 1000 / sampleRate;
     auto start = std::chrono::steady_clock::now();
     auto callbacks = sp<OffloadCallbacks>::make();
     std::mutex presentationEndLock;