MPEG4Extractor:avoid comparing different int types

samplerate and track.timescale are of different int types. Explicit
cast is needed.

Bug: 117681690

Test: make cts && cts-tradefed run cts-dev -m \
CtsMediaTestCases --compatibility:module-arg \
CtsMediaTestCases:include-annotation:\
android.platform.test.annotations.RequiresDevice

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.

Change-Id: Ic654b5ba3260fb3297be0824e4a01d229905a448