Work around MDL button bug in iOS 13.
Work around MDL button bug in iOS 13.

In iOS 13, buttons with mdl ripple containers are broken;
seemingly, iOS 13 does not like the container having a default
opacity of 0.
This changes the default opacity to 0.01.

Fixes #2376

Change-Id: I0218d3911cce1c4b776ca6debf51ff62e4bbe4f3

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