Bug 1054520 - Add ifdef to enable stumbler in nightly. r=nalexander

mobile/android/confvars.sh

@@ -86,5 +86,9 @@ if test ! "$RELEASE_BUILD"; then
   MOZ_ANDROID_SHARE_OVERLAY=1
 fi
 
-# Don't enable the Mozilla Location Service stumbler.
-# MOZ_ANDROID_MLS_STUMBLER=1
+# Enable the Mozilla Location Service stumbler in Nightly.
+if test "$NIGHTLY_BUILD"; then
+  MOZ_ANDROID_MLS_STUMBLER=1
+else
+  MOZ_ANDROID_MLS_STUMBLER=
+fi