Refactor audio sample calculation for EAC3

[minlinbroadcom]

Calculate the number of audio samples based on frame size and number of frames for EAC3.

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[FongMi]

For #3276, the sample count calculation can overcount when an EC-3 sample contains multiple syncframes for different substreams.

In Ac3Util.java, the new code derives numFrames from buffer.remaining() / firstFrameBytes and returns numFrames * samplesPerFrame. However, in ISO BMFF, an E-AC-3 sample may contain multiple syncframes for different substreams or shorter block frames, and those syncframes do not necessarily represent consecutive presentation time. Per ETSI TS 102 366 Annex F, the EC-3 sample duration is still six audio blocks, i.e. 1536 PCM samples.

For example, a sample with two substreams and three audio blocks per syncframe may contain four syncframes. This implementation would return 4 * 768 = 3072, but the correct sample duration is still 1536. I think this should either be reverted or implemented as MP4/EC-3-specific logic, rather than changing a utility method whose contract is to parse one syncframe and relying on buffer.remaining().