Add margins between the buttons

... esp on tablets.

Also define the Timer text size as 70dp instead of 70sp to fit the space,
independent of the user's font size setting.

TODO: Make the buttons taller on tablets.
Tablets: BIG timer text in the activity
remove button_drawable_padding

since these buttons no longer have text labels that need padding from
the button icons. This makes the left-right sides uniform.
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.