MPEG4Extractor:media timescale for audio delay/pad

segmentDuration elst entry ('edts') in mp4 file is based on timescale in movie header('mvhd').
But mediaTime is based on media header timescale ('mdhd').(Part 12:
ISO base media file format). Use media header timescale for mediaTime in delay/pad calculation.

Bug: 117681690
Bug: 80513071
Bug: 118053992
Bug: 118248950

Test: played failing clips from referenced bugs and gapless playback test files using mediaplayer
	and observed expected behavior. Verified calculated delay/padding samples with
	logcat messages manually.

CtsMediaTestCases passed:
	MediaExtractorTest
	MediaPlayerTest
	MediaRecorderTest
	MediaSyncTest
	MediaMuxerTest
	MediaRandomTest

Change-Id: I2c55fec365009c1580ed1967b1b0ba5db19928d2