vold: restore -D__ANDROID_DEBUGGABLE__

Regression introduced by
commit 7ebcc2bcfd
("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

Android.bp

@@ -146,6 +146,11 @@ cc_library_static {
         "model/VolumeBase.cpp",
         "model/VolumeEncryption.cpp",
     ],
+    product_variables: {
+        debuggable: {
+            cppflags: ["-D__ANDROID_DEBUGGABLE__"],
+        },
+    },
     shared_libs: [
         "android.hardware.health.storage@1.0",
     ],