Annnotate libcore.io.Memory as @SystemApi

Bug: 157639992
Test: m droid
Change-Id: I201510d3c5205305228ac6358f19ae13d31cf421
diff --git a/api/system-current.txt b/api/system-current.txt
new file mode 100644
index 0000000..d802177
--- /dev/null
+++ b/api/system-current.txt
@@ -0,0 +1 @@
+// Signature format: 2.0