TTML: Support referential styling #2953

Issue description

Observed: MP4-Encapsulated TTML subtitles default to top position (with positionAnchor defaulting to Float.MIN_VALUE)
Expected: Subtitles appear at the bottom of the screen with a valid position

Reproduction steps

Using a dash steam w/ MP4-Encapsulated TTML subtitles, enable subtitles. Subtitles appear at the top.

Link to test content

Cannot provide (legal reasons)

Version of ExoPlayer being used

2.4.1 (Was not an issue in 2.4.0)

Device(s) and version(s) of Android being used

Device: FireTV
OS: Fire OS v5.2.4.1 (Android v5.1.1)