Expand on how to customize the UI in the tutorial.
Expand on how to customize the UI in the tutorial.

Apparently, the UI customization tutorial is a bit hard
for people to understand. This adds some more context
to the code examples.

Issue #2241.

Change-Id: I6218e4afb8293dfc1cc98e3677868f36f27cec09

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