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
diff --git a/Android.mk b/Android.mk
index b813d48..4d2074e 100644
--- a/Android.mk
+++ b/Android.mk
@@ -1,5 +1,5 @@
 # Copyright (C) 2015 The CyanogenMod Project
-# Copyright (C) 2017-2019 The LineageOS Project
+# Copyright (C) 2017-2020 The LineageOS Project
 #
 # Licensed under the Apache License, Version 2.0 (the "License");
 # you may not use this file except in compliance with the License.
@@ -14,6 +14,8 @@
 # limitations under the License.
 LOCAL_PATH := $(call my-dir)
 
+include $(CLEAR_VARS)
+
 # We have a special case here where we build the library's resources
 # independently from its code, so we need to find where the resource
 # class source got placed in the course of building the resources.