vold: restore -D__ANDROID_DEBUGGABLE__

Regression introduced by
commit 7ebcc2bcfde2cf6713e2975224034b06ba0943d8
("Revert "[Vold] Conditional dependency to ARC++ ObbVolume"")
errantly removed the product_variables support for
-D__ANDROID_DEBUGGABLE__ when built for userdebug builds.

Restore stanza.

Signed-off-by: Mark Salyzyn <salyzyn@google.com>
Bug: 161454607
Test: adb-remount-test no longer reports a problem
Change-Id: Ia7e55d1174fa7fac2d934d4aac06d15e8be388af
diff --git a/Android.bp b/Android.bp
index 2c47859..039f60e 100644
--- a/Android.bp
+++ b/Android.bp
@@ -146,6 +146,11 @@
         "model/VolumeBase.cpp",
         "model/VolumeEncryption.cpp",
     ],
+    product_variables: {
+        debuggable: {
+            cppflags: ["-D__ANDROID_DEBUGGABLE__"],
+        },
+    },
     shared_libs: [
         "android.hardware.health.storage@1.0",
     ],