MPEG4Extractor: Use ssize_t to correctly handle read errors

Change the type of `bytesRead` from size_t to ssize_t to properly
handle negative return values from the readAt() function, which
indicate an I/O error.

FLAG: com.android.media.extractor.flags.extractor_mp4_enable_iamf
Bug: 428836838

Test: atest MediaExtractorTest
Change-Id: Ic78a70c90a31588407e6d798ae586ea805dc32ac