Use the right system property name.

Bug: 22358539
Change-Id: I0bf9719a2b54acbde80f3c911988724581447b0c
diff --git a/Ext4Crypt.cpp b/Ext4Crypt.cpp
index 028c12c..8bf03cb 100644
--- a/Ext4Crypt.cpp
+++ b/Ext4Crypt.cpp
@@ -52,7 +52,7 @@
 
 using android::base::StringPrintf;
 
-static const char* kPropEmulateFbe = "persist.vold.emulate_fbe";
+static const char* kPropEmulateFbe = "persist.sys.emulate_fbe";
 
 namespace {
     // Key length in bits