Drop the [no]proguard. prefix for the intermediate targets

Since we have enabled Progaurd for all build variants, we don't need the
different naming scheme along the path to make incremental build work
properly when you switch build variant.
As a result, this fixes the nothing-to-do-do-build-gets-something-build bug
introduced by the multi-dex change.

Change-Id: I907ff13091013d28910b0aa190b9218144385132