apex: unmount flattened apex directory before installing zips

Change-Id: Idd1cadedcab94e81e30a487bd09f9173e5fe577e
diff --git a/twrpApex.cpp b/twrpApex.cpp
index 45e3e30..5531445 100755
--- a/twrpApex.cpp
+++ b/twrpApex.cpp
@@ -45,6 +45,7 @@
 			LOGERR("Unable to bind mount flattened apex directory\n");
 			return false;
 		}
+		android::base::SetProperty("twrp.apex.flattened", "true");
 		return true;
 	}
 	if (!mountApexOnLoopbackDevices(apexFiles)) {