TIF: Make selected tracks identifiable

A recent change on the multi-track API surfaced an issue that it's hard to
identify selected tracks from the list of tracks obtained from
TvView.getTracks() method, especially when two or more tracks convey the same
track information (e.g. two English stereo audio tracks with the same sample
rate). This change introduced a new public method getId() in TvTrackInfo that
returns an ID with which one can identify the tracks uniquely. It also led to
minor signature changes across the multi-track API to make it more
developer-friendly in both application and TV input service developers'
perspectives.

Change-Id: I226f75e697b0ce52b44db7471660f6d9f646f6f9