Fix ChooseSdkVersion after api levels

I2954bb21c1cfdeb305f25cfb6c8711c930f6ed50 switched normalizeVersions
to work on ApiLevels, which inadvertantly caused it to return "current"
instead of "10000" for libraries that specify "current" in their stubs
property.  ChooseSdkVersion couldn't handle "current" because it was
manually converting the version to an int.  Switch ChooseSdkVersion
to use ApiLevels instead so that it can handle "current".

Test: m checkbuild
Change-Id: Id412359e092483ba419118dd03bc206fae702a96