Bug 830644 - Support WVGA and other non-HiDPI screen dimensions and resolutions

Changes on `shared/style` and `shared/style_unstable` css for bigger assets support (background-size declarations mostly)
  * Potential issues will raise with this changes
  * I will like some help on testing this branch and identifying the issues
  * This PR will not land until that applications land their own PR (for scaling) to avoid this regressions

Added @2x assests for all the shared branding and style folders
  * There are some missing ones, @sergivila  will take care on generating it.

Currently affected applications:
  * **sms** (clear button too bigger)
  * **browser** (some buttons images too bigger)

Please @rnowm @sergivila @jmcanterafonseca @timdream @rexboy7 take a look!