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/runas.te b/microdroid/sepolicy/system/private/runas.te
new file mode 100644
index 0000000..ef31aac
--- /dev/null
+++ b/microdroid/sepolicy/system/private/runas.te
@@ -0,0 +1,4 @@
+typeattribute runas coredomain;
+
+# ndk-gdb invokes adb shell run-as.
+domain_auto_trans(shell, runas_exec, runas)