Tablets: Big buttons in the activity

* Add ic_{pause, play, replay, stop}.svg images.
* Add a script to export them as ic_*_big.png at multiple resolutions.
* Check in the exported .png files.
* Add ic_*.xml resources linking to the notification icons ic_action_*
  at phone screen sizes and to the new ic_*_big drawables at tablet screen
  sizes.
* Switch to ic_pause et al drawables in MainActivity.
* Update 7" tablet screenshot.
* Add 10" tablet screenshots.

* Also reduce button_padding a little.