Respect LOCAL_MIN_SDK_VERSION for dx

Follow Jack's behavior for now and pass LOCAL_MIN_SDK_VERSION to
dx.  Don't pass it to desugar for now, desugar with low
--min_sdk_version numbers enables features we haven't verified
yet, and existing apps won't be using the new language features
that require extra desugaring.

Test: examine classes.dex from an app with LOCAL_MIN_SDK_VERSION
Change-Id: Ic7d679b1b11fba98a33418a4f79d4762a91f1f9c