Handle channel count parsing for AC-4 with discrete objects

[ybai001]

Some AC-4 stream consists of sub stream with discrete objects. Due to limitations in the current AC-4 specification (ETSI TS 103 190-2 V1.2.1 (2018-02)), discrete object number information is not explicitly stated in the ISO Base Media File Format (Annex E). To prevent exception triggering, "Maximum number of tracks if audio presentation includes object audio" in table 77 of ETSI TS 103 190-2 V1.2.1 (2018-02) is used as the channel count.

This pull request provides the corresponding implementation in AC4 parser.