MPEG4Extractor: ensure returned status is checked.

Also fix handling of zero atom size in MPEG4Source::parseChunk.
IDataSource: ensure readAt returns correct status.
Test: manually test with mediaplayer.
Bug: 34718515
Change-Id: I1219ec579aa0876dc1230e36af46b158b84c6d77

(cherry picked from commit ff1fb4d5cdd3b2b28c69edd8cd3021e335ca381a)
(cherry picked from commit 371561214467f848496928914f771703d6c331e6)

Change-Id: Idfbee6317dfea21a526a0522f65fbcb67d92ab60