Add file contexts for com.android.extservices APEX.

Bug: 147674123
Test: built on aosp_taimen and verified package is available from the
apex.
Change-Id: I416b3267c116b18b204dd63f999b127f52b1757a
diff --git a/apex/com.android.extservices-file_contexts b/apex/com.android.extservices-file_contexts
new file mode 100644
index 0000000..f3a65d4
--- /dev/null
+++ b/apex/com.android.extservices-file_contexts
@@ -0,0 +1 @@
+(/.*)?                u:object_r:system_file:s0