Add microdroid specific sepolicy

Microdroid will have a separate sepolicy, apart from the core policy.
This is the first step; For now it's a simple copy of system/sepolicy.
For the future work, it will be stripped.

Bug: 189165759
Test: boot microdroid and see selinux enforced
Change-Id: I2fee39f7231560b49c93bd5e8d0feeffada40938
diff --git a/microdroid/sepolicy/system/private/users b/microdroid/sepolicy/system/private/users
new file mode 100644
index 0000000..51b7b57
--- /dev/null
+++ b/microdroid/sepolicy/system/private/users
@@ -0,0 +1 @@
+user u roles { r } level s0 range s0 - mls_systemhigh;