Add system/core/Android.bp

Test: Soong's build.ninja is identical before/after (except GlobRules)
Change-Id: I97c1e4cedad1dfc8622abb6cfed6257971199a96
diff --git a/Android.bp b/Android.bp
new file mode 100644
index 0000000..b44c296
--- /dev/null
+++ b/Android.bp
@@ -0,0 +1 @@
+subdirs = ["*"]