Remove mention of lpreview from BUILDING.md

Removed lpreview build section
Updated gradle commands (removed lpreview and classic productFlavors)
Updated SDK Manager packages needed (removed kiktkat, included lollipop)

SDK Build-tools version 21 has solved and unsolved issues, Current rev is 21.0.1
https://code.google.com/p/android/issues/detail?id=77629
http://stackoverflow.com/questions/26446610/android-gradle-project-upgrading-build-tools-to-21-0-1-aapt-throws-exception