Add support for preview versions of platforms.

Add support for preview versions of platforms.

ro.build.version.codename is a new property indicating whether a platform
is in its release form (value = REL) or in development (value = dev branch
name such as Donut). When the codename indicates a development/preview version
then the API level must be ignored and this codename is used as a unique
identifier of the platform.

IAndroidTarget has been changed to return an instance of a new class
AndroidVersion instead of the api level directly. This class helps deals with
the logic of comparing version from targets or devices.

This change impacts all of the sdk manager to deal with targets identified by
codename instead of api level. This in turn impacts everything that relies
on the sdkmanager: ADT (build, launch, project creation), the AVD manager,
the SDK updater.