Don't attempt to write to / for aar prebuilts

This path needs to be set if we're going to build aar prebuilts with
aapt2.

Bug: 33381544
Test: Attempt to build an aar prebuilt with LOCAL_USE_AAPT2 := true, it
      doesn't fail writing to /

Change-Id: I28c85736986221607a1d7a295ff1522b993a2049
Merged-In: I28c85736986221607a1d7a295ff1522b993a2049
(cherry picked from commit 48845257dc9fcf6cd753aeb3548829177525394e)
diff --git a/core/prebuilt_internal.mk b/core/prebuilt_internal.mk
index 5c9d822..d9ae234 100644
--- a/core/prebuilt_internal.mk
+++ b/core/prebuilt_internal.mk
@@ -535,7 +535,7 @@
 full_android_manifest :=
 my_res_resources :=
 my_overlay_resources :=
-my_compiled_res_base_dir :=
+my_compiled_res_base_dir := $(intermediates.COMMON)/flat-res
 R_file_stamp :=
 proguard_options_file :=
 my_generated_res_dirs := $(intermediates.COMMON)/aar/res