Adds support for building apps on API Level 4

   - Increases minSDK to 4 in Compiler.java (takes App Inventor out of
     screen compatibility mode).
   - Adds a checkbox to preview the app on tablet or phone size.
   - Adds correction for resolution of images depending on density of
     device
   - changes pixels for DPs
   - Adds scaleToFit property to Image component

Change-Id: I2938a2254f59eb534785afd37ec8891d9148e190