fix a few bugs in the studio download button behavior:

 - make button text always white (avoid blue on blue during hover)
 - revise logic to add the platform name to download button to avoid weird duplicity bug on small screens
 - remove the animation script when clicking download before agree because it reloaded the page (unkown cause)
depends on: https://googleplex-android-review.git.corp.google.com/#/c/752757/

Change-Id: I1b2b4ff9c0b0cbbd34f1c77a2b736fe3425a16e4