Add microdroid_file_contexts to build image

This file is a simple copy of system/sepolicy/private/file_contexts to
support contexts for microdroid.img. It contains unnecessary entries,
but we'll leave it for now and clean up after enough experiment.

Bug: 178993690
Test: boot and see selinux denials are gone
Change-Id: I43782937a282b752091640b249288415137db6b6
diff --git a/microdroid/Android.bp b/microdroid/Android.bp
index fd2c6c2..526be7d 100644
--- a/microdroid/Android.bp
+++ b/microdroid/Android.bp
@@ -6,4 +6,5 @@
     deps: [
         "init_second_stage",
     ],
+    file_contexts: "microdroid_file_contexts",
 }