Always use installed version of d8-compat-dx

Also adapt to the renaming of d8 to d8-compat-dx.

Test: USE_D8=true m -j16 PRODUCT-aosp_arm64-eng showcommands core-oj
Change-Id: Ic7aaa6a5da34a6a5706c20f53922a83cb1de1100
diff --git a/root.bp b/root.bp
index 6788fa7..bc59171 100644
--- a/root.bp
+++ b/root.bp
@@ -29,6 +29,7 @@
     "packages/apps/*",
     "prebuilts/clang/host/linux-x86",
     "prebuilts/ndk",
+    "prebuilts/r8",
     "prebuilts/sdk",
     "prebuilts/misc",
     "system/*",