Add SOONG_SDK_SNAPSHOT_PREFER support

By default the generated snapshot has prefer: false. Building it with
SOONG_SDK_SNAPSHOT_PREFER=true will force it to generate prefer: true.

Bug: 157884619
Test: m nothing
      m SOONG_SDK_SNAPSHOT_PREFER=true art-module-sdk
      - check the generated Android.bp file to make sure it contains
        prefer: true
Change-Id: Ied297b32d9bd4822a140fa99016f38e234c50f64