Add 'dash' as a compatible brand

Per the spec, the 'dash' brand indicates support for the concatenation
of an Initialization Segment and a single Indexed Media Segment. Since
we support this, add 'dash' as a supported brand to improve compatibility.

Bug: 63390273
Test: played 'dash' brand file
Change-Id: Ifb38337200857f190085bda030434d7f56a817f3