Move microdroid sepolicy to system/sepolicy

Bug: 190511750
Test: boot microdroid
Change-Id: I4aa4a56e9be5103d70469c3508110a973f3e4f12
diff --git a/microdroid/reqd_mask/mls b/microdroid/reqd_mask/mls
new file mode 100644
index 0000000..d276924
--- /dev/null
+++ b/microdroid/reqd_mask/mls
@@ -0,0 +1 @@
+mlsconstrain binder { set_context_mgr } (l1 eq l2);