Fix overlapping nested cues in UI text display
Fix overlapping nested cues in UI text display

The position: 'absolute' style should only be set when other
positioning attributes (left, right, top, bottom) are being set.
Otherwise, the natural layout flow is disturbed, without anything to
replace it.

Closes #2524

Change-Id: I4062c68253218a0f4ace6560a8847381d80c48d5

tvoli/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player