Fix mainDexClasses cleaning.

Use of exec was preventing normal continuation of the script
execution.

Change-Id: I617b1f215dddd586b905f61c48b702fa285ea5bd
diff --git a/dx/etc/mainDexClasses b/dx/etc/mainDexClasses
index 154a4df..034d47e 100644
--- a/dx/etc/mainDexClasses
+++ b/dx/etc/mainDexClasses
@@ -155,4 +155,4 @@
   -libraryjars "${shrinkedAndroidJar}" -dontoptimize -dontobfuscate -dontpreverify \
   -include "${baserules}" 1>/dev/null || exit 10
 
-exec java -cp "$jarpath" com.android.multidex.ClassReferenceListBuilder "${tmpOut}" ${@} ||  exit 11
+java -cp "$jarpath" com.android.multidex.ClassReferenceListBuilder "${tmpOut}" ${@} ||  exit 11