lineage-sdk: Clear variables before attempting to generate res R/Manifest.java

* Fixes really strange looking errors while building rild:
  FAILED: ninja: 'out/target/product/generic_arm64/vendor/bin/hw/org.lineageos.platform-res.apk', needed by 'out/target/product/generic_arm64/vendor/bin/hw/rild', missing and no known rule to make it

Change-Id: I0fb91383829b9b478b3dee9686e9e719ea3ed695