Remove mention of lpreview from BUILDING.md (and update build instructions)

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