Use prebuild R8 for USE_R8=true

Instead of building R8 from external/r8 use the prebuild jar in
prebuilds/r8.

Test: m -j USE_R8=true

Change-Id: I9680ae430cdbc2b3beb910873c8eed27354f6af1
diff --git a/core/config.mk b/core/config.mk
index 1692f2f..cc3a2f0 100644
--- a/core/config.mk
+++ b/core/config.mk
@@ -531,7 +531,6 @@
   SIGNAPK_JAR := $(HOST_OUT_JAVA_LIBRARIES)/signapk$(COMMON_JAVA_PACKAGE_SUFFIX)
   SIGNAPK_JNI_LIBRARY_PATH := $(HOST_OUT_SHARED_LIBRARIES)
   ZIPALIGN := $(HOST_OUT_EXECUTABLES)/zipalign
-  R8 := $(HOST_OUT_EXECUTABLES)/r8
 
 else # TARGET_BUILD_APPS || TARGET_BUILD_PDK
   AIDL := $(prebuilt_sdk_tools_bin)/aidl
@@ -542,9 +541,10 @@
   SIGNAPK_JAR := $(prebuilt_sdk_tools)/lib/signapk$(COMMON_JAVA_PACKAGE_SUFFIX)
   SIGNAPK_JNI_LIBRARY_PATH := $(prebuilt_sdk_tools)/$(HOST_OS)/lib64
   ZIPALIGN := $(prebuilt_sdk_tools_bin)/zipalign
-  R8 := $(prebuilt_build_tools_wrappers)/r8
 endif # TARGET_BUILD_APPS || TARGET_BUILD_PDK
 
+R8_COMPAT_PROGUARD_JAR := prebuilts/r8/compatproguard-master.jar
+
 ifeq (,$(TARGET_BUILD_APPS))
   # Use RenderScript prebuilts for unbundled builds but not PDK builds
   LLVM_RS_CC := $(HOST_OUT_EXECUTABLES)/llvm-rs-cc